diff --git a/src/Gateway.php b/src/Gateway.php
index 1b54a18..ed7484a 100644
--- a/src/Gateway.php
+++ b/src/Gateway.php
@@ -85,6 +85,35 @@ public function __construct(string $id, string $token, string $environment)
         $this->environment = $environment;
     }
 
+    /**
+     * Capture a pre-authorized a transaction.
+     *
+     * @param \CraigPaul\Moneris\Transaction|string $transaction
+     * @param string|null $order
+     *
+     * @return \CraigPaul\Moneris\Response
+     */
+    public function capture($transaction, string $order = null, $amount = null)
+    {
+        if ($transaction instanceof Transaction) {
+            $order = $transaction->order();
+            $amount = $amount ?? $transaction->amount();
+            $transaction = $transaction->number();
+        }
+
+        $params = [
+            'type' => 'completion',
+            'crypt_type' => Crypt::SSL_ENABLED_MERCHANT,
+            'comp_amount' => $amount,
+            'txn_number' => $transaction,
+            'order_id' => $order,
+        ];
+
+        $transaction = $this->transaction($params);
+
+        return $this->process($transaction);
+    }
+
     /**
      * Pre-authorize a purchase.
      *
diff --git a/src/Transaction.php b/src/Transaction.php
index 87cc41a..c53b034 100644
--- a/src/Transaction.php
+++ b/src/Transaction.php
@@ -230,6 +230,12 @@ public function valid()
                     $errors[] = Validator::set($params, 'order_id') ? null : 'Order id not provided.';
                     $errors[] = Validator::set($params, 'txn_number') ? null : 'Transaction number not provided.';
 
+                    break;
+                case 'completion':
+                    $errors[] = Validator::set($params, 'comp_amount') ? null : 'Amount not provided.';
+                    $errors[] = Validator::set($params, 'order_id') ? null : 'Order id not provided.';
+                    $errors[] = Validator::set($params, 'txn_number') ? null : 'Transaction number not provided.';
+
                     break;
                 case 'refund':
                     $errors[] = Validator::set($params, 'amount') ? null : 'Amount not provided.';