The move towards higher production pressures and temperatures means that seal design for downhole usage may become a limiting factor for the exploitation of future wells. To determine factors associated with seal behavior under severe (HP/HT) conditions, a dynamic seal test facility has been built to simulate downhole conditions up to 15,000psi and 250°C. A number of novel seal designs have been developed and tested for HP/HT service in downhole completions using this test facility. An important aspect of test procedure is whether the test is performed under absolute or differential pressure conditions. Previous work has emphasised absolute pressure conditions. This paper compares absolute and differential conditions and investigates which of these represents the more severe case.