diff --git a/y2023/localizer/localizer.h b/y2023/localizer/localizer.h
new file mode 100644
index 0000000..b89d222
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+++ b/y2023/localizer/localizer.h
@@ -0,0 +1,121 @@
+#ifndef Y2023_LOCALIZER_LOCALIZER_H_
+#define Y2023_LOCALIZER_LOCALIZER_H_
+
+#include <array>
+#include <map>
+
+#include "frc971/constants/constants_sender_lib.h"
+#include "frc971/control_loops/drivetrain/hybrid_ekf.h"
+#include "frc971/control_loops/drivetrain/improved_down_estimator.h"
+#include "frc971/control_loops/drivetrain/localization/localizer_output_generated.h"
+#include "frc971/control_loops/drivetrain/localization/utils.h"
+#include "frc971/imu_reader/imu_watcher.h"
+#include "frc971/vision/target_map_generated.h"
+#include "y2023/constants/constants_generated.h"
+#include "y2023/localizer/status_generated.h"
+#include "y2023/localizer/visualization_generated.h"
+
+namespace y2023::localizer {
+
+class Localizer {
+ public:
+  static constexpr size_t kNumCameras = 4;
+  typedef Eigen::Matrix<double, 4, 4> Transform;
+  typedef frc971::control_loops::drivetrain::HybridEkf<double> HybridEkf;
+  typedef HybridEkf::State State;
+  typedef HybridEkf::Output Output;
+  typedef HybridEkf::Input Input;
+  typedef HybridEkf::StateIdx StateIdx;
+  Localizer(aos::EventLoop *event_loop,
+            const frc971::control_loops::drivetrain::DrivetrainConfig<double>
+                dt_config);
+
+ private:
+  class Corrector : public HybridEkf::ExpectedObservationFunctor {
+   public:
+    // Indices used for each of the members of the output vector for this
+    // Corrector.
+    enum OutputIdx {
+      kX = 0,
+      kY = 1,
+      kTheta = 2,
+    };
+    Corrector(const State &state_at_capture, const Eigen::Vector3d &Z)
+        : state_at_capture_(state_at_capture), Z_(Z) {
+      H_.setZero();
+      H_(kX, StateIdx::kX) = 1;
+      H_(kY, StateIdx::kY) = 1;
+      H_(kTheta, StateIdx::kTheta) = 1;
+    }
+    Output H(const State &, const Input &) final;
+    Eigen::Matrix<double, HybridEkf::kNOutputs, HybridEkf::kNStates> DHDX(
+        const State &) final {
+      return H_;
+    }
+
+   private:
+    Eigen::Matrix<double, HybridEkf::kNOutputs, HybridEkf::kNStates> H_;
+    const State state_at_capture_;
+    const Eigen::Vector3d &Z_;
+  };
+
+  struct CameraState {
+    aos::Sender<Visualization> debug_sender;
+    Transform extrinsics = Transform::Zero();
+    aos::util::ArrayErrorCounter<RejectionReason, RejectionCount>
+        rejection_counter;
+    size_t total_candidate_targets = 0;
+    size_t total_accepted_targets = 0;
+  };
+
+  static std::array<CameraState, kNumCameras> MakeCameras(
+      const Constants &constants, aos::EventLoop *event_loop);
+  flatbuffers::Offset<TargetEstimateDebug> HandleTarget(
+      int camera_index, const aos::monotonic_clock::time_point capture_time,
+      const frc971::vision::TargetPoseFbs &target,
+      flatbuffers::FlatBufferBuilder *debug_fbb);
+  void HandleImu(aos::monotonic_clock::time_point sample_time_pico,
+                 aos::monotonic_clock::time_point sample_time_pi,
+                 std::optional<Eigen::Vector2d> encoders, Eigen::Vector3d gyro,
+                 Eigen::Vector3d accel);
+  flatbuffers::Offset<TargetEstimateDebug> RejectImage(
+      int camera_index, RejectionReason reason,
+      TargetEstimateDebug::Builder *builder);
+
+  void SendOutput();
+  flatbuffers::Offset<frc971::control_loops::drivetrain::LocalizerState>
+  PopulateState(flatbuffers::FlatBufferBuilder *fbb) const;
+  flatbuffers::Offset<ImuStatus> PopulateImu(
+      flatbuffers::FlatBufferBuilder *fbb) const;
+  void SendStatus();
+
+  aos::EventLoop *const event_loop_;
+  const frc971::control_loops::drivetrain::DrivetrainConfig<double> dt_config_;
+  frc971::constants::ConstantsFetcher<Constants> constants_fetcher_;
+  std::array<CameraState, kNumCameras> cameras_;
+  const std::array<Transform, kNumCameras> camera_extrinsics_;
+  const std::map<uint64_t, Transform> target_poses_;
+
+  frc971::control_loops::drivetrain::DrivetrainUkf down_estimator_;
+  HybridEkf ekf_;
+  HybridEkf::ExpectedObservationAllocator<Corrector> observations_;
+
+  frc971::controls::ImuWatcher imu_watcher_;
+  frc971::control_loops::drivetrain::LocalizationUtils utils_;
+
+  aos::Sender<Status> status_sender_;
+  aos::Sender<frc971::controls::LocalizerOutput> output_sender_;
+  aos::monotonic_clock::time_point t_ = aos::monotonic_clock::min_time;
+  size_t clock_resets_ = 0;
+
+  size_t total_candidate_targets_ = 0;
+  size_t total_accepted_targets_ = 0;
+
+  // For the status message.
+  std::optional<Eigen::Vector2d> last_encoder_readings_;
+};
+
+// Converts a TargetPoseFbs into a transformation matrix.
+Localizer::Transform PoseToTransform(const frc971::vision::TargetPoseFbs *pose);
+}  // namespace y2023::localizer
+#endif  // Y2023_LOCALIZER_LOCALIZER_H_