Bank reserves in the United States increased dramatically at the end of 2008. Subsequent asset purchase programs in 2009 and 2011 more than doubled the quantity of reserves outstanding. These events required major adjustments in banks' balance sheets. We study the evolution of reserve holdings across banks from the fall of 2008 until the middle of 2011 and document how banks' balance sheets changed concurrently. Motivated by the potential implications for monetary policy of operating with a high level of reserves, we focus particular attention on those banks which accumulated large quantities of reserves.