This seminar analyzes the relation between Ricardo, Sraffa, and Marx on the questions surrounding the theory of value; and develops the necessary tools for dealing with the theory of prices and output in a realistic multi-sectoral economy. It begins by analyzing (conceptually and mathematically) Leontief's attempt to reconstruct the classical theory of output and Sraffa's attempt to reconstruct the classical theory of prices. It continues by examining various developments of Sraffa's work and attempts to assess their theoretical and empirical significance with regards to (1) estimating the long-period prices and output of an economy, and (2) solving the puzzles of value (and the infamous 'transformation' problem) in Ricardo and Marx. Finally, it addresses contemporary issues, where the classical theory of prices and output may shed new light: namely, (1) the explanation of technological change through competitive processes; (2) the theories of comparative and absolute cost advantage theories of international trade, and (3) the determination of interest rates.