Although his classic work has gone through many reprintings and translations, only now has Paul A. Samuelson added new material to his 1947 treatise. A new introduction portrays the genesis of the book and analyzes how its contributions fit into theoretical developments of the last thirty-five years. A new and lengthy mathematical appendix gives a survey of the following post-1947 breakthroughs in political economy, in relation to the methodology of Foundations: linear programming and comparative statics; nonlinear programming, dynamic and stochastic; modern duality theory; the testable content of the neoclassical money model; probabilistic decision making, with new slants on the dogma of Expected-Utility maximizing; and portfolio and liquidity preference analysis by general methods that transcend mean-variance approximations.