Ricardo's book, On the Principles of Political Economy and Taxation, was first published in 1817 (London: John Murray, Albemarle-Street), with second and third editions in quick succession.We present Ricardo's final revision, the third edition, published in 1821, here.The three different editions encompassed several substantive changes in the development of Ricardo's ideas. A comprehensive, readable comparison of the three editions can be found Works of David Ricardo, Vol. 1, ed. by Pierro Sraffa with the collaboration of M. H. Dobb, Cambridge: Cambridge University Press, 1951. We are indebted to this fine work and have relied on it to correct occasional typographical misprints in the 1821 edition.Minor editorial modifications in this edition...