This SAE Aerospace Recommended Practice (ARP) is an application guide for hydraulic power transfer units, and describes the various types, typical design approaches, their operational characteristics and limitations, circuit recommendations, and a tabulation of typical applications. The scope is limited to devices that transfer power between hydraulic systems and do so by means of rotary subassemblies such as hydraulic motors and pumps. The purpose of this document is to apprise the system designer of the available options in power transfer units, configuration tradeoffs, and system design precautions in their application.