The first ever NATO Advanced Study Institute devoted to ＇Thermodynamics and the Optimization of Complex Energy Systems＇ was held in July 1998 in Neptun, Romania. The ASI unified the field by bringing together leading researchers from engineering and physics, and from West and East. This note outlines the objectives and proceedings of the Institute, which reviewed the most recent advances in exergy analysis, entropy-generation minimization and thermoeconomics. Directions for future developments (e.g. constructal theory) an also outlined.