diff --git a/include/aare/PixelHistogram.hpp b/include/aare/PixelHistogram.hpp
index ae72f15..3a3560e 100644
--- a/include/aare/PixelHistogram.hpp
+++ b/include/aare/PixelHistogram.hpp
@@ -1,12 +1,16 @@
 #pragma once
 #include "aare/NDArray.hpp"
 #include "aare/NDView.hpp"
+#include "aare/ProducerConsumerQueue.hpp"
 
 //Lets see if we need to hide it behind a pimpl
 #include <boost/histogram.hpp>
-#include <cstdint>
 #include <atomic>
+#include <chrono>
 #include <condition_variable>
+#include <cstddef>
+#include <cstdint>
+#include <memory>
 #include <mutex>
 #include <thread>
 #include <vector>
@@ -25,6 +29,8 @@ class PixelHistogram {
         bh::axis::integer<int, bh::use_default, bh::axis::option::none_t>,
         bh::axis::integer<int, bh::use_default, bh::axis::option::none_t>>;
     using Hist = bh::histogram<Axes, bh::dense_storage<StorageType>>;
+    using AsyncQueue = ProducerConsumerQueue<NDArray<AxisType, 2>>;
+
     int rows_;
     int cols_;
     int n_threads_;
@@ -47,15 +53,52 @@ class PixelHistogram {
     std::atomic<bool> stop_workers_;
     int work_generation_;
 
+    // Serialises calls into the synchronous fan-out (`fill`). The
+    // coordinator thread acquires it for the duration of each item it
+    // processes, and direct callers of `fill` also acquire it. Without
+    // this, concurrent sync + async fills would race on `current_image_`
+    // and `work_generation_`.
+    std::mutex fill_mutex_;
+
+    // Async producer/consumer pipeline. SPSC queue feeds the coordinator
+    // thread, which calls the synchronous `fill` one image at a time.
+    std::unique_ptr<AsyncQueue> async_queue_;
+    std::thread coordinator_;
+    std::atomic<bool> stop_coordinator_{false};
+    std::atomic<bool> coordinator_busy_{false};
+    std::chrono::microseconds async_wait_{100};
+
     // Private worker thread method
     void worker_loop(int thread_id);
+    void coordinator_loop();
     int row_start(int thread_id) const;
     int row_count(int thread_id) const;
 
   public:
-    PixelHistogram(int rows, int cols, int n_bins, double xmin, double xmax, int n_threads = 1);
+    PixelHistogram(int rows, int cols, int n_bins, double xmin, double xmax,
+                   int n_threads = 1, std::size_t max_pending = 16);
     ~PixelHistogram();
+
+    // Synchronous fill: blocks until the image has been merged into the
+    // accumulators. Safe to call concurrently with `fill_async` (calls are
+    // serialised through `fill_mutex_`).
     void fill(const NDView<AxisType, 2> &image);
+
+    // Asynchronous fill: takes ownership of `image`, enqueues it for the
+    // coordinator thread, and returns. Blocks the caller only if the queue
+    // is full (single-producer, single-consumer queue with a sleep-poll
+    // backpressure loop, matching the convention in ClusterFinderMT).
+    void fill_async(NDArray<AxisType, 2> image);
+
+    // Wait for all queued async fills to complete. Cheap when the queue
+    // is already drained.
+    void flush() const;
+
+    // Number of items either queued or currently being processed.
+    std::size_t pending() const;
+
+    // Implicitly flushes pending async fills first so the snapshot is
+    // consistent with everything that was submitted up to the call.
     NDArray<StorageType, 3> hdata() const;
     NDArray<AxisType, 1> bin_centers() const;
     NDArray<AxisType, 1> bin_edges() const;
diff --git a/python/aare/__init__.py b/python/aare/__init__.py
index f4bddda..cadeb01 100644
--- a/python/aare/__init__.py
+++ b/python/aare/__init__.py
@@ -43,3 +43,4 @@ from ._aare import apply_calibration, count_switching_pixels
 from ._aare import calculate_pedestal, calculate_pedestal_float, calculate_pedestal_g0, calculate_pedestal_g0_float
 
 from ._aare import VarClusterFinder
+from ._aare import PixelHistogram
diff --git a/python/src/bind_PixelHistogram.hpp b/python/src/bind_PixelHistogram.hpp
index cdbf6aa..cf87e7a 100644
--- a/python/src/bind_PixelHistogram.hpp
+++ b/python/src/bind_PixelHistogram.hpp
@@ -13,10 +13,10 @@ using namespace ::aare;
 void define_pixel_histogram_bindings(py::module &m) {
     py::class_<PixelHistogram>(m, "PixelHistogram",
                                 "A histogram for pixel-wise statistics")
-        .def(py::init<int, int, int, double, double, int>(),
+        .def(py::init<int, int, int, double, double, int, std::size_t>(),
              R"(
              Initialize a PixelHistogram.
-             
+
              Args:
                  rows: Number of rows in the detector
                  cols: Number of columns in the detector
@@ -24,9 +24,13 @@ void define_pixel_histogram_bindings(py::module &m) {
                  xmin: Minimum value for histogram range
                  xmax: Maximum value for histogram range
                  n_threads: Number of threads for parallel filling (default: 1)
+                 max_pending: Maximum number of images that can be queued for
+                     asynchronous filling before fill_async() applies
+                     backpressure on the caller (default: 16)
              )",
              py::arg("rows"), py::arg("cols"), py::arg("n_bins"),
-             py::arg("xmin"), py::arg("xmax"), py::arg("n_threads") = 1)
+             py::arg("xmin"), py::arg("xmax"), py::arg("n_threads") = 1,
+             py::arg("max_pending") = std::size_t{16})
 
         .def("fill",
              [](PixelHistogram &self,
@@ -35,21 +39,75 @@ void define_pixel_histogram_bindings(py::module &m) {
                  self.fill(view);
              },
              R"(
-             Fill the histogram with image data.
-             
+             Fill the histogram with image data (blocking).
+
              Args:
                  image: A 2D numpy array of pixel values (dtype: float32)
              )",
              py::arg("image").noconvert())
 
+        .def("fill_async",
+             [](PixelHistogram &self,
+                py::array_t<PixelHistogram::AxisType, 0> image) {
+                 // Copy the numpy buffer into an owned NDArray while we
+                 // still hold the GIL so we don't depend on the array's
+                 // backing storage outliving this call.
+                 auto view = make_view_2d(image);
+                 NDArray<PixelHistogram::AxisType, 2> owned(view);
+                 // Release the GIL while enqueueing - fill_async can block
+                 // on backpressure when the queue is full.
+                 py::gil_scoped_release release;
+                 self.fill_async(std::move(owned));
+             },
+             R"(
+             Submit an image for asynchronous filling.
+
+             The image is copied into an internal buffer before this call
+             returns, so the caller may mutate or free the numpy array
+             immediately. The actual histogram update happens on a
+             background thread. If the internal queue is full this call
+             blocks (with the GIL released) until a slot becomes available.
+
+             Args:
+                 image: A 2D numpy array of pixel values (dtype: float32)
+             )",
+             py::arg("image").noconvert())
+
+        .def("flush",
+             &PixelHistogram::flush,
+             R"(
+             Block until all images submitted via fill_async() have been
+             merged into the accumulators. Cheap when nothing is pending.
+             )",
+             py::call_guard<py::gil_scoped_release>())
+
+        .def("pending",
+             &PixelHistogram::pending,
+             R"(
+             Return the number of images either waiting in the queue or
+             currently being processed by the background thread. Useful
+             for monitoring/diagnostics.
+             )")
+
         .def("hdata",
              [](const PixelHistogram &self) {
-                 auto ptr = new NDArray<PixelHistogram::StorageType, 3>(self.hdata());
+                 // hdata() implicitly flushes - release the GIL while it
+                 // does so. Allocation/copy into the NDArray runs without
+                 // the GIL too; only the numpy wrapping needs it.
+                 NDArray<PixelHistogram::StorageType, 3>* ptr = nullptr;
+                 {
+                     py::gil_scoped_release release;
+                     ptr = new NDArray<PixelHistogram::StorageType, 3>(self.hdata());
+                 }
                  return return_image_data(ptr);
              },
              R"(
              Get the histogram data as a numpy array.
-             
+
+             Implicitly flushes any pending asynchronous fills before
+             returning, so the snapshot is consistent with everything
+             submitted up to this call.
+
              Returns:
                  A 3D numpy array containing the histogram bins for each pixel
              )")
@@ -61,7 +119,7 @@ void define_pixel_histogram_bindings(py::module &m) {
              },
              R"(
              Get the bin centers along the value axis.
-             
+
              Returns:
                  A 1D numpy array containing the center values for each histogram bin
              )")
@@ -72,7 +130,7 @@ void define_pixel_histogram_bindings(py::module &m) {
              },
              R"(
              Get the bin edges along the value axis.
-             
+
              Returns:
                  A 1D numpy array containing the edge values for the histogram bins
              )");
diff --git a/src/PixelHistogram.cpp b/src/PixelHistogram.cpp
index 228a225..a0aadfd 100644
--- a/src/PixelHistogram.cpp
+++ b/src/PixelHistogram.cpp
@@ -5,13 +5,17 @@
 #include <boost/histogram/storage_adaptor.hpp>
 #include <boost/histogram/unsafe_access.hpp>
 #include <cstring>
+#include <exception>
+#include <iostream>
 #include <stdexcept>
+#include <utility>
 #include <vector>
 
 namespace aare {
 
 
-PixelHistogram::PixelHistogram(int rows, int cols, int n_bins, double xmin, double xmax, int n_threads):
+PixelHistogram::PixelHistogram(int rows, int cols, int n_bins, double xmin, double xmax,
+                               int n_threads, std::size_t max_pending):
              rows_(rows), cols_(cols), n_threads_(n_threads), xmin_(xmin), xmax_(xmax), current_image_(nullptr),
              completed_threads_(0), stop_workers_(false), work_generation_(0) {
     if (rows_ < 1 || cols_ < 1 || n_bins < 1) {
@@ -20,6 +24,9 @@ PixelHistogram::PixelHistogram(int rows, int cols, int n_bins, double xmin, doub
     if (n_threads < 1) {
         throw std::invalid_argument("PixelHistogram requires at least one thread");
     }
+    if (max_pending < 1) {
+        throw std::invalid_argument("PixelHistogram requires max_pending >= 1");
+    }
 
     n_threads_ = std::min(n_threads_, rows_);
 
@@ -54,16 +61,29 @@ PixelHistogram::PixelHistogram(int rows, int cols, int n_bins, double xmin, doub
     for (int i = 0; i < n_threads_; ++i) {
         workers_.emplace_back([this, i]() { this->worker_loop(i); });
     }
+
+    // Async pipeline. The PCQ holds (size - 1) usable slots, so size up by
+    // one to honour the requested max_pending.
+    async_queue_ = std::make_unique<AsyncQueue>(static_cast<std::uint32_t>(max_pending + 1));
+    coordinator_ = std::thread([this]() { this->coordinator_loop(); });
 }
 
 PixelHistogram::~PixelHistogram() {
+    // Drain any pending async fills before tearing down the worker pool.
+    // The coordinator's loop keeps processing while stop_coordinator_ is
+    // true as long as the queue is non-empty (mirrors ClusterFinderMT).
+    if (coordinator_.joinable()) {
+        stop_coordinator_ = true;
+        coordinator_.join();
+    }
+
     // Signal all workers to stop
     {
         std::unique_lock<std::mutex> lock(work_mutex_);
         stop_workers_ = true;
     }
     work_cv_.notify_all();
-    
+
     // Join all worker threads
     for (auto& thread : workers_) {
         if (thread.joinable()) {
@@ -126,6 +146,10 @@ void PixelHistogram::worker_loop(int thread_id) {
 }
 
 NDArray<PixelHistogram::StorageType, 3> PixelHistogram::hdata() const {
+    // Make sure any pending async fills are merged in before we snapshot
+    // the partial histograms. Cheap when the queue is already drained.
+    flush();
+
     const auto &hist = partial_hists_.front();
     const auto bins = static_cast<ssize_t>(hist.axis(0).size());
     const auto cols = static_cast<ssize_t>(hist.axis(1).size());
@@ -159,6 +183,10 @@ void PixelHistogram::fill(const NDView<AxisType, 2> &image) {
         throw std::invalid_argument("PixelHistogram image shape does not match constructor shape");
     }
 
+    // Serialise all calls into the fan-out. fill_mutex_ is always the
+    // outermost lock; work_mutex_ is taken briefly inside it.
+    std::lock_guard<std::mutex> fill_lock(fill_mutex_);
+
     // Reset counters and set work
     {
         std::unique_lock<std::mutex> lock(work_mutex_);
@@ -166,10 +194,10 @@ void PixelHistogram::fill(const NDView<AxisType, 2> &image) {
         current_image_ = &image;
         ++work_generation_;
     }
-    
+
     // Signal all worker threads to start
     work_cv_.notify_all();
-    
+
     // Wait for all workers to complete
     {
         std::unique_lock<std::mutex> lock(work_mutex_);
@@ -178,6 +206,56 @@ void PixelHistogram::fill(const NDView<AxisType, 2> &image) {
     }
 }
 
+void PixelHistogram::fill_async(NDArray<AxisType, 2> image) {
+    if (image.shape(0) != rows_ || image.shape(1) != cols_) {
+        throw std::invalid_argument("PixelHistogram image shape does not match constructor shape");
+    }
+
+    // SPSC backpressure: spin with a short sleep until a slot frees up.
+    // The std::move only consumes `image` on the iteration that succeeds
+    // (placement-new inside write() runs only when the slot is free).
+    while (!async_queue_->write(std::move(image))) {
+        std::this_thread::sleep_for(async_wait_);
+    }
+}
+
+void PixelHistogram::flush() const {
+    while (!async_queue_->isEmpty() || coordinator_busy_.load(std::memory_order_acquire)) {
+        std::this_thread::sleep_for(async_wait_);
+    }
+}
+
+std::size_t PixelHistogram::pending() const {
+    // sizeGuess() counts the items still in the queue; the coordinator
+    // does `read()` (which pops) before setting `coordinator_busy_`, so an
+    // in-flight item lives only in the busy flag.
+    return async_queue_->sizeGuess() +
+           (coordinator_busy_.load(std::memory_order_acquire) ? 1u : 0u);
+}
+
+void PixelHistogram::coordinator_loop() {
+    NDArray<AxisType, 2> item;
+    while (!stop_coordinator_.load(std::memory_order_acquire) || !async_queue_->isEmpty()) {
+        if (async_queue_->read(item)) {
+            coordinator_busy_.store(true, std::memory_order_release);
+            try {
+                fill(item.view());
+            } catch (const std::exception& e) {
+                // fill_async pre-validates shape, so this is purely
+                // defensive. Log to stderr and keep the coordinator alive.
+                std::cerr << "PixelHistogram::fill_async error: "
+                          << e.what() << std::endl;
+            } catch (...) {
+                std::cerr << "PixelHistogram::fill_async error: unknown"
+                          << std::endl;
+            }
+            coordinator_busy_.store(false, std::memory_order_release);
+        } else {
+            std::this_thread::sleep_for(async_wait_);
+        }
+    }
+}
+
 NDArray<PixelHistogram::AxisType, 1> PixelHistogram::bin_centers() const {
     const auto& value_axis = partial_hists_.front().axis(0);
     const auto n_bins = static_cast<ssize_t>(value_axis.size());
diff --git a/src/PixelHistogram.test.cpp b/src/PixelHistogram.test.cpp
index f79dfb7..1d8fdbe 100644
--- a/src/PixelHistogram.test.cpp
+++ b/src/PixelHistogram.test.cpp
@@ -1,8 +1,10 @@
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/generators/catch_generators.hpp>
 
+#include <chrono>
 #include <cstdint>
 #include <random>
+#include <thread>
 #include <vector>
 
 #include "test_config.hpp"
@@ -206,3 +208,123 @@ TEST_CASE("Random fills match a reference implementation") {
     }
     CHECK(all_match);
 }
+
+TEST_CASE("Async fill matches sync fill") {
+    // Submit a stream of random frames through fill_async and compare
+    // against the same frames processed by fill() on a separate histogram.
+    constexpr int rows = 19;
+    constexpr int cols = 23;
+    constexpr int n_bins = 16;
+    constexpr float xmin = -1.0f;
+    constexpr float xmax = 3.0f;
+    const int n_threads = GENERATE(1, 2, 4);
+    constexpr int n_frames = 32;
+    // Pick a small queue capacity so the producer trips the backpressure
+    // path at least a few times during the run.
+    constexpr std::size_t max_pending = 4;
+
+    PixelHistogram async_hist(rows, cols, n_bins, xmin, xmax, n_threads, max_pending);
+    PixelHistogram sync_hist(rows, cols, n_bins, xmin, xmax, n_threads);
+
+    std::mt19937 rng(0xA5A5A5A5);
+    std::uniform_real_distribution<float> dist(xmin - 0.25f, xmax + 0.25f);
+
+    for (int f = 0; f < n_frames; ++f) {
+        NDArray<float, 2> img({rows, cols}, 0.0f);
+        for (ssize_t r = 0; r < rows; ++r)
+            for (ssize_t c = 0; c < cols; ++c)
+                img(r, c) = dist(rng);
+        sync_hist.fill(img.view());
+        async_hist.fill_async(std::move(img));
+    }
+    // hdata() calls flush() internally, but exercise the explicit path too.
+    async_hist.flush();
+    CHECK(async_hist.pending() == 0);
+
+    auto a = async_hist.hdata();
+    auto s = sync_hist.hdata();
+    REQUIRE(a.shape(0) == s.shape(0));
+    REQUIRE(a.shape(1) == s.shape(1));
+    REQUIRE(a.shape(2) == s.shape(2));
+
+    bool all_match = true;
+    for (ssize_t r = 0; r < rows && all_match; ++r) {
+        for (ssize_t c = 0; c < cols && all_match; ++c) {
+            for (ssize_t b = 0; b < n_bins && all_match; ++b) {
+                if (a(r, c, b) != s(r, c, b)) {
+                    all_match = false;
+                    INFO("r=" << r << " c=" << c << " b=" << b
+                         << " async=" << a(r, c, b) << " sync=" << s(r, c, b));
+                    CHECK(a(r, c, b) == s(r, c, b));
+                }
+            }
+        }
+    }
+    CHECK(all_match);
+}
+
+TEST_CASE("fill_async with mismatched shape throws") {
+    PixelHistogram hist(8, 8, 16, 0.0, 1.0, 2);
+    NDArray<float, 2> bad({4, 4}, 0.0f);
+    CHECK_THROWS_AS(hist.fill_async(std::move(bad)), std::invalid_argument);
+}
+
+TEST_CASE("Destructor drains pending async fills") {
+    // Submit more frames than the queue can hold so backpressure kicks in,
+    // then immediately let the histogram go out of scope and verify that
+    // the merged hdata() matches the reference computed sequentially.
+    constexpr int rows = 11;
+    constexpr int cols = 7;
+    constexpr int n_bins = 8;
+    constexpr float xmin = 0.0f;
+    constexpr float xmax = 1.0f;
+    constexpr int n_frames = 12;
+    constexpr std::size_t max_pending = 2;
+
+    std::vector<NDArray<float, 2>> frames;
+    frames.reserve(n_frames);
+    std::mt19937 rng(0xDEADBEEF);
+    std::uniform_real_distribution<float> dist(xmin, xmax);
+    for (int f = 0; f < n_frames; ++f) {
+        NDArray<float, 2> img({rows, cols}, 0.0f);
+        for (ssize_t r = 0; r < rows; ++r)
+            for (ssize_t c = 0; c < cols; ++c)
+                img(r, c) = dist(rng);
+        frames.push_back(std::move(img));
+    }
+
+    NDArray<aare::PixelHistogram::StorageType, 3> snapshot({rows, cols, n_bins}, uint16_t{0});
+    {
+        PixelHistogram hist(rows, cols, n_bins, xmin, xmax, 2, max_pending);
+        for (auto& img : frames) {
+            // Move a copy so we can also build the reference below.
+            NDArray<float, 2> copy({rows, cols}, 0.0f);
+            std::memcpy(copy.data(), img.data(), copy.total_bytes());
+            hist.fill_async(std::move(copy));
+        }
+        // No explicit flush(); destructor must drain.
+        // Capture hdata() *after* the loop but inside the scope so it
+        // observes everything that was submitted (hdata flushes too).
+        snapshot = hist.hdata();
+    }
+
+    PixelHistogram reference(rows, cols, n_bins, xmin, xmax, 2);
+    for (const auto& img : frames) reference.fill(img.view());
+    auto expected = reference.hdata();
+
+    bool all_match = true;
+    for (ssize_t r = 0; r < rows && all_match; ++r) {
+        for (ssize_t c = 0; c < cols && all_match; ++c) {
+            for (ssize_t b = 0; b < n_bins && all_match; ++b) {
+                if (snapshot(r, c, b) != expected(r, c, b)) {
+                    all_match = false;
+                    INFO("r=" << r << " c=" << c << " b=" << b
+                         << " got=" << snapshot(r, c, b)
+                         << " expected=" << expected(r, c, b));
+                    CHECK(snapshot(r, c, b) == expected(r, c, b));
+                }
+            }
+        }
+    }
+    CHECK(all_match);
+}