diff --git a/frc971/control_loops/index.cc b/frc971/control_loops/index.cc
new file mode 100644
index 0000000..845f1c6
--- /dev/null
+++ b/frc971/control_loops/index.cc
@@ -0,0 +1,237 @@
+#include "frc971/control_loops/index.h"
+
+#include <stdio.h>
+
+#include <algorithm>
+
+#include "aos/aos_core.h"
+
+#include "aos/common/messages/RobotState.q.h"
+#include "aos/common/control_loop/control_loops.q.h"
+#include "aos/common/logging/logging.h"
+
+#include "frc971/constants.h"
+#include "frc971/control_loops/index_motor_plant.h"
+
+using ::aos::time::Time;
+
+namespace frc971 {
+namespace control_loops {
+
+IndexMotor::IndexMotor(control_loops::IndexLoop *my_index)
+    : aos::control_loops::ControlLoop<control_loops::IndexLoop>(my_index),
+      wrist_loop_(new StateFeedbackLoop<2, 1, 1>(MakeIndexLoop())),
+      hopper_disc_count_(0),
+      total_disc_count_(0),
+      loader_up_(false),
+      disc_clamped_(false),
+      disc_ejected_(false),
+      last_bottom_disc_detect_(false) {
+}
+
+const /*static*/ double IndexMotor::kDiscRadius = 10.875 * 0.0254 / 2;
+const /*static*/ double IndexMotor::kRollerRadius = 2.0 * 0.0254 / 2;
+
+bool IndexMotor::FetchConstants() {
+  if (!constants::horizontal_lower_limit(&horizontal_lower_limit_)) {
+    LOG(ERROR, "Failed to fetch the horizontal lower limit constant.\n");
+    return false;
+  }
+  if (!constants::horizontal_upper_limit(&horizontal_upper_limit_)) {
+    LOG(ERROR, "Failed to fetch the horizontal upper limit constant.\n");
+    return false;
+  }
+  if (!constants::horizontal_hall_effect_start_angle(
+          &horizontal_hall_effect_start_angle_)) {
+    LOG(ERROR, "Failed to fetch the horizontal start angle constant.\n");
+    return false;
+  }
+  if (!constants::horizontal_zeroing_speed(
+          &horizontal_zeroing_speed_)) {
+    LOG(ERROR, "Failed to fetch the horizontal zeroing speed constant.\n");
+    return false;
+  }
+
+  return true;
+}
+
+// Distance to move the indexer when grabbing a disc.
+const double kNextPosition = 10.0;
+
+/*static*/ double IndexMotor::ConvertDiscAngleToIndex(const double angle) {
+  return (angle * (1 + (kDiscRadius * 2 + kRollerRadius) / kRollerRadius));
+}
+
+/*static*/ double IndexMotor::ConvertDiscAngleToDiscPosition(const double angle) {
+  return angle * (kDiscRadius + kRollerRadius);
+}
+
+/*static*/ double IndexMotor::ConvertIndexToDiscAngle(const double angle) {
+  return (angle / (1 + (kDiscRadius * 2 + kRollerRadius) / kRollerRadius));
+}
+
+/*static*/ double IndexMotor::ConvertIndexToDiscPosition(const double angle) {
+  return IndexMotor::ConvertDiscAngleToDiscPosition(
+      ConvertIndexToDiscAngle(angle));
+}
+
+// Positive angle is towards the shooter, and positive power is towards the
+// shooter.
+void IndexMotor::RunIteration(
+    const control_loops::IndexLoop::Goal *goal,
+    const control_loops::IndexLoop::Position *position,
+    control_loops::IndexLoop::Output *output,
+    control_loops::IndexLoop::Status *status) {
+  // Make goal easy to work with.
+  Goal goal_enum = static_cast<Goal>(goal->goal_state);
+
+  // Disable the motors now so that all early returns will return with the
+  // motors disabled.
+  if (output) {
+    output->transfer_voltage = 0.0;
+    output->index_voltage = 0.0;
+  }
+
+  status->ready_to_intake = false;
+
+  // Cache the constants to avoid error handling down below.
+  if (!FetchConstants()) {
+    return;
+  }
+
+  if (position) {
+    wrist_loop_->Y << position->index_position;
+  }
+  const double index_position = wrist_loop_->X_hat(0, 0);
+
+  bool safe_to_change_state_ = true;
+  switch (safe_goal_) {
+    case HOLD:
+      // The goal should already be good, so sit tight with everything the same
+      // as it was.
+      printf("HOLD Not implemented\n");
+      break;
+    case READY_LOWER:
+      printf("READY_LOWER Not implemented\n");
+      break;
+    case INTAKE:
+      {
+        Time now = Time::Now();
+        if (hopper_disc_count_ < 4) {
+          output->transfer_voltage = 12.0;
+        }
+        // Posedge of the disc entering the beam break.
+        if (position) {
+          if (position->bottom_disc_detect && !last_bottom_disc_detect_) {
+            transfer_frisbee_.Reset();
+            transfer_frisbee_.bottom_posedge_time_ = now;
+            printf("Posedge of bottom disc %f\n",
+                   transfer_frisbee_.bottom_posedge_time_.ToSeconds());
+            ++hopper_disc_count_;
+          }
+
+          // Disc exited the beam break now.
+          if (!position->bottom_disc_detect && last_bottom_disc_detect_) {
+            transfer_frisbee_.bottom_negedge_time_ = now;
+            printf("Negedge of bottom disc %f\n",
+                   transfer_frisbee_.bottom_negedge_time_.ToSeconds());
+            frisbees_.push_front(transfer_frisbee_);
+          }
+
+          if (position->bottom_disc_detect) {
+            output->transfer_voltage = 12.0;
+            // Must wait until the disc gets out before we can change state.
+            safe_to_change_state_ = false;
+
+            // TODO(aschuh): A disc on the way through needs to start moving the
+            // indexer if it isn't already moving.  Maybe?
+
+            Time elapsed_posedge_time = now -
+                transfer_frisbee_.bottom_posedge_time_;
+            if (elapsed_posedge_time >= Time::InSeconds(0.3)) {
+              // It has been too long.  The disc must be jammed.
+              LOG(ERROR, "Been way too long.  Jammed disc?\n");
+              printf("Been way too long.  Jammed disc?\n");
+            }
+          }
+
+          for (Frisbee &frisbee : frisbees_) {
+            if (!frisbee.has_been_indexed_) {
+              output->transfer_voltage = 12.0;
+              Time elapsed_posedge_time = now -
+                  frisbee.bottom_posedge_time_;
+              if (elapsed_posedge_time >= Time::InSeconds(0.07)) {
+                // Should have just engaged.
+                // Save the indexer position, and the time.
+
+                // It has been long enough since the disc entered the indexer.
+                // Treat now as the time at which it contacted the indexer.
+                LOG(INFO, "Grabbed on the index now at %f\n", index_position);
+                printf("Grabbed on the index now at %f\n", index_position);
+                frisbee.has_been_indexed_ = true;
+                frisbee.index_start_position_ = index_position;
+                frisbee.index_start_time_ = now;
+              }
+            }
+            if (!frisbee.has_been_indexed_) {
+              // Discs must all be indexed before it is safe to stop indexing.
+              safe_to_change_state_ = false;
+            }
+          }
+
+          double new_index_position = wrist_loop_->R(0, 0);
+
+          // TODO(aschuh): As we loop through, assess the state of the indexer
+          // and figure if the bottom disc is in a place such that we can
+          // intake without filling the hopper early.
+          // status->ready_to_intake = false;
+
+          for (Frisbee &frisbee : frisbees_) {
+            if (frisbee.has_been_indexed_) {
+              // We want to store it pi from where the disc was grabbed
+              // (for now).
+              new_index_position = ::std::max(
+                  new_index_position,
+                  (frisbee.index_start_position_ +
+                   ConvertDiscAngleToIndex(M_PI)));
+              // TODO(aschuh): We should be able to pick the M_PI knowing if
+              // the next disc is coming in hot or not.
+            }
+          }
+          wrist_loop_->R << new_index_position, 0.0;
+        }
+        printf("INTAKE Not implemented\n");
+      }
+      break;
+    case READY_SHOOTER:
+      printf("READY_SHOOTER Not implemented\n");
+      break;
+    case SHOOT:
+      printf("SHOOT Not implemented\n");
+      break;
+  }
+
+  // Update the observer.
+  wrist_loop_->Update(position != NULL, output == NULL);
+
+  if (position) {
+    LOG(DEBUG, "pos=%f currently %f\n",
+        position->index_position, index_position);
+    last_bottom_disc_detect_ = position->bottom_disc_detect;
+  }
+
+  status->hopper_disc_count = hopper_disc_count_;
+  status->total_disc_count = total_disc_count_;
+
+
+  if (output) {
+    output->index_voltage = wrist_loop_->U(0, 0);
+  }
+
+  if (safe_to_change_state_) {
+    safe_goal_ = goal_enum;
+  }
+}
+
+}  // namespace control_loops
+}  // namespace frc971