The impact of operation in deep water on the pressure rating of equipment is a special concern. The objective of this document is to foster a better understanding of the effects of simultaneous internal and external pressures on the rated working pressure of equipment covered by the scope of API Specification 17D@ Subsea Wellhead and Tree Equipment. Additionally@ it is intended to provide a high-level overview of issues that should be considered if a user elects to consider differential pressure in their design@ especially in components with irregular geometry and with high stress concentrations. It is not intended to serve as a design specification. This document was prepared in response to a request from the API Subcommittee 17 (SC17).