This paper discusses the development of 8000 psi aircraft lightweight hydraulic systems as compared to the present 3000 psi systems. The concept of utilizing higher operating pressures for aircraft hydraulic system provides for significant reductions in both weight and volume. The power level of hydraulic systems in military aircraft has risen from less than 10 hp in the early 1940s to nearly 300 on the Navy's modern F-14 fighter, 500 on the SST Concorde, and 1000 on the Air Force's B-1 bomber. With considerable theoretical and empirical data available, a selection of 8000 psi was determined to be the best practical level to operate an aircraft hydraulic system. The development of aircraft Lightweight Hydraulic Systems (LHS) has been an ongoing Navy R&D program for the past 10 years and is being funded by the Naval Air Development Center (NADC) at Warminster, PA. Two major areas that provide the impetus to operate at this level were the elastomeric O-ring seal and the advent of a reliable variable volume displacement type pump. No significant change in the operating pressure level has been made since that time. However, the Navy and Rockwell have recognized that considerable advances in the state-of-the-art have been made and higher operating pressures are now feasible.