The National Aeronautics and Space Administration (NASA) is engaged in a Supersonic Cruise Airplane Research (SCAR) Program to study and experimentally evaluate advanced technology that is applicable to future supersonic transports. A broad spectrum of advanced engine concepts including Variable Cycle Engines (VCEs) has been evaluated by Pratt & Whitney Aircraft (P&WA) in conjunction with the SCAR Program. This paper describes some of the advanced engine concepts and VCEs that were evaluated, and summarizes the features and problems of each engine. Based on the results from this study, one engine has been identified as being the most promising, the Variable Stream Control Engine (VSCE). It has the potential to significantly reduce jet noise and to improve airplane performance relative to first-generation supersonic transport (SST) engines. Future propulsion programs that are required for the United States to reach a competitive position in the advanced supersonic commercial airplane market are also reviewed.