diff --git a/Civi/Api4/Action/ContactCategory/GetFlows.php b/Civi/Api4/Action/ContactCategory/GetFlows.php
new file mode 100644
index 0000000..2a9d98f
--- /dev/null
+++ b/Civi/Api4/Action/ContactCategory/GetFlows.php
@@ -0,0 +1,167 @@
+<?php
+namespace Civi\Api4\Action\ContactCategory;
+
+use CRM_Contactcats_ExtensionUtil as E;
+use Civi;
+use Civi\Api4\Generic\Result;
+use CRM_Core_DAO;
+use CRM_Core_Exception;
+use DateTimeImmutable;
+
+/**
+ * Get counts of contacts moving between categories between two given dates.
+ *
+ * @see \Civi\Api4\Generic\AbstractAction
+ *
+ */
+class GetFlows extends \Civi\Api4\Generic\AbstractAction {
+
+  /**
+   * Start Date
+   *
+   * @required
+   * @var string
+   */
+  protected string $startDate;
+
+  /**
+   * End Date (optional)
+   *
+   * @var string|NULL
+   * @default NULL
+   */
+  protected ?string $endDate = NULL;
+
+  public function _run(Result $result) {
+    Civi::log()->debug('Begin', ['=start' => 'GetFlows', '=timed' => 1, 'from' => $this->startDate, 'to' => $this->endDate]);
+
+    // Check dates are valid and get a Ymd format of the day *following* the specified one.
+    // This is so we can find activities *before* this date and thereby capture activities that
+    // occurred *on* the given date.
+    $getValidDate = fn(?string $input) => $input ? (new DateTimeImmutable($input))->modify('+1 day')->format('Ymd'): FALSE;
+
+    $startDate = $getValidDate($this->startDate);
+    if (!$startDate) {
+      throw new CRM_Core_Exception(E::ts("Cannot parse given start date."));
+    }
+    $endDate = $getValidDate($this->endDate);
+    if (!$endDate) {
+      throw new CRM_Core_Exception(E::ts("Cannot parse given end date."));
+    }
+    if (!($endDate > $startDate)) {
+      throw new CRM_Core_Exception(E::ts("This version of CiviCRM does not support twisting the space time continuum. End date cannot be before start date."));
+    }
+
+    $result['windowFunctionsSupported'] = $this->windowFunctionsSupported();
+    if ($result['windowFunctionsSupported']) {
+      $this->solveWithWindowFunctions($result, $startDate, $endDate);
+    }
+    else {
+      // TODO: make a slow version that doesn't need window functions.
+      // $this->solveWithoutWindowFunctions($result);
+      throw new CRM_Core_Exception("your database does not support Window functions.");
+    }
+
+    Civi::log()->debug("Complete.", ['=' => 'set', 'result' => $result->getArrayCopy()]);
+  }
+
+  /**
+   * Does the database support window functions in the way we need?
+   *
+   * MariaDB 10.2+ does.
+   */
+  protected function windowFunctionsSupported() {
+    $cache = \CRM_Utils_Cache::create(['type' => ['SqlGroup'], 'name' => 'contactcats']);
+    $supported = $cache->get('windowFunctionsSupported'); // Will return default if cached value expired.
+    $supported = null;
+    if ($supported === NULL) {
+      // Need to test for window functions.
+      $supported = FALSE;
+      // Prevent Civi handling possible execption
+      $handler = set_exception_handler(fn() => false);
+      try {
+        \CRM_Core_DAO::executeQuery("CREATE TEMPORARY TABLE contactcats_window_check (i int unsigned not null, x int unsigned not null)");
+        \CRM_Core_DAO::executeQuery("INSERT INTO contactcats_window_check VALUES (1, 1), (2, 1), (3, 2);");
+        $rows = \CRM_Core_DAO::executeQuery("SELECT x, ROW_NUMBER() OVER ( PARTITION BY (x) ORDER BY i DESC) rn FROM contactcats_window_check ORDER BY x, rn")->fetchAll();
+        $supported = ($rows === [ [ 'x' => "1", 'rn' => "1"], [ 'x' => "1", 'rn' => "2"], [ 'x' => "2", 'rn' => "1"] ]) ;
+      }
+      catch (\Exception $e) {
+        // Silent fail.
+      }
+      // Restore exception handler
+      set_exception_handler($handler);
+      $cache->set('windowFunctionsSupported', $supported);
+    }
+    return $supported;
+  }
+
+  /**
+   *
+   */
+  protected function solveWithWindowFunctions(Result $result, string $startDateYmd, string $endDateYmd) {
+
+    $sql = <<<SQL
+    /* Identify the relevant activities for the contacts */
+    WITH activities AS (
+      SELECT ac.contact_id, a.activity_date_time, a.id activity_id
+      FROM civicrm_activity a
+      INNER JOIN civicrm_activity_contact ac 
+        ON a.id = ac.activity_id AND ac.record_type_id = 3
+      INNER JOIN civicrm_contact_category cc 
+        ON cc.id = ac.contact_id
+      WHERE a.activity_type_id = 89
+    ),
+
+    /* activities1 is the latest activity before the window's start date */
+    activities1 AS (
+        SELECT contact_id, activity_id,
+          ROW_NUMBER() OVER (
+            PARTITION BY (contact_id)
+            ORDER BY activity_date_time DESC
+          ) rn
+        FROM activities a1
+        WHERE a1.activity_date_time < %1
+        ),
+
+    /* activities1 is the latest activity before the window's end date */
+    activities2 AS (
+        SELECT contact_id, activity_id,
+          ROW_NUMBER() OVER (
+            PARTITION BY (contact_id)
+            ORDER BY activity_date_time DESC
+          ) rn
+        FROM activities a2
+        WHERE a2.activity_date_time < %2
+        )
+
+    /* join activities1 and 2 to count changes between each shift */
+    SELECT cat1.new_category_id from_category_id, cat2.new_category_id to_category_id, count(*) contact_count
+    FROM activities2 
+    INNER JOIN civicrm_value_category_chan_41 cat1
+      ON activities2.activity_id = cat1.entity_id
+    LEFT JOIN (
+      activities1
+      INNER JOIN civicrm_value_category_chan_41 cat2
+        ON activities1.activity_id = cat2.entity_id
+        )
+      ON activities1.contact_id = activities2.contact_id AND activities1.rn = 1
+    WHERE activities2.rn = 1
+    GROUP BY from_category_id, to_category_id
+    ;
+    SQL;
+
+    $data = CRM_Core_DAO::executeQuery($sql, [
+      1 => [$endDateYmd, 'Int'],
+      2 => [$startDateYmd, 'Int'],
+    ])->fetchAll();
+
+    // Don't use exchange array so as not to gazump non-array data(?)
+    foreach ($data as $row) {
+      $result[] = $row;
+    }
+  }
+
+
+
+}
+