Luis Antonio Santalo (Spain 1911 - Argentina 2001) contributed to several branches of Geometry, his laying of the mathematical foundations of Stereology and its applications perhaps being his most outstanding achievement. A considerable power of abstraction, a brilliant geometric intuition and an outstanding gift as a disseminator of science were among his virtues. The present volume contains a selection of his best papers. Part I consists of a short biography and some photographs together with a complete list of his publications, classified into research papers, books, and articles on education and the popularization of mathematics, as well as a comprehensive analysis of his contribution to science. Part II, the main part of the book, includes selected papers, arranged into five sections according to the nature of their contents: Differential Geometry, Integral Geometry, Convex Geometry, Affine Geometry, and Statistics and Stereology. Each section is preceded by a commentary by a renowned specialist: Teufel, Langevin, Schneider, Leichtweiss, and Cruz-Orive, respectively. Finally, Part III emphasizes the influence of his work. It contains commentaries by several specialists regarding modern results based on, or closely related to, those of Santalo, some book reviews written by Santalo, as well as some reviews of his books. As a curious addendum, a ranking of his articles, given by Santalo himself, is included.