Alberto Calderón was one of the leading mathematicians of the twentieth century. His fundamental, pioneering work reshaped the landscape of mathematical analysis. This volume presents a wide selection from some of Calderón's most influential papers. They range from singular integrals to partial differential equations, from interpolation theory to Cauchy integrals on Lipschitz curves, from inverse problems to ergodic theory. The depth, originality, and historical impact of these works are vividly illustrated by the accompanying commentaries by some of today's leading figures in analysis. In addition, two biographical chapters preface the volume. They discuss Alberto Calderón's early life and his mathematical career.