"This technical report addresses issues related to the effects of external pressure acting on API Subcommittee 17 (SC17@ Subcommittee on Subsea Production Equipment) subsea equipment installed in deepwater for containing or controlling wellbore fluids. External pressure at deepwater can significantly reduce the differential pressure acting on the wall of subsea equipment@ and therefore@ this can improve its internal pressure containment capability. External pressure is typically ambient seawater pressure@ but in some cases@ external pressure may be due to the hydrostatic head of drilling mud@ completion fluids@ or other fluids contained within risers or other conduits that connect the subsea equipment to surface facilities. There is a need for guidelines on the application of external pressure during the design@ validation and operation of subsea equipment. Guidelines are also needed to calculate and/or determine a modification to the working pressure limits at the installed water depth@ using the selected equipment API rated working pressure (RWP). API Technical Report 17TR12 (hereafter API 17TR12) provides guidance for subsea equipment designers/ manufacturers to properly account for external pressure (or in some cases@ differential pressure) when designing and validating subsea equipment. Additionally@ this technical report provides guidance to equipment purchaser/end-user to appropriately select rated equipment for their subsea systems with consideration to the effects of external pressure in addition to internal pressure@ including differential pressure across a closure mechanism@ and other applied mechanical or structural loads under all potential operating scenarios and functionality criteria. NOTE API Technical Report 17TR4 (hereafter API 17TR4) provides additional information on the effects of external pressure on stresses generated within subsea equipment for the equipment designer. API 17TR12 applies specifically to API SC17 equipment. API 17TR12 is to be used as a supplement to the equipment's applicable API product specification (e.g. API 6A@ API 17D@ API 17G)@ depending on its specific application@ associated regulations@ and project requirements. Other API product specifications may elect to adopt this technical report@ subject to their component hardware@ application-related design constraints and acceptance criteria. Specific subsea recommended practices@ standards@ and/or specifications may elect to adopt this technical report@ also subject to their component hardware and application-related design constraints. For this technical report@ the term ""equipment"" also applies to the terms ""part""@ ""component""@ ""sub-component"" or ""device"" within a subsea system."