Santaló (Spain 1911 - Argentina 2001) contributed to several branches of Geometry, perhaps his most outstanding achievement being his having laid the mathematical foundations of Stereology and its applications. Considerable power of abstraction, a brilliant geometric intuition and his outstanding gifts as a disseminator of science were among his virtues. The present volume contains a selection of his best papers edited by Antonio M. Naveira and Augustí Reventós in collaboration with Graciela S. Birman and Ximo Gual-Arnau. Part I consists of a short biography and some photographs along 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 the selected papers, arranged in 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 comment by a renowned specialist: Teufel, Langevin, Schneider, Leichtweiss, and Cruz-Orive respectively. Finally, Part III emphasizes the influence of his work. It contains comments by several specialists about modern results based on, or closely related to, those of Santaló, some book reviews written by Santaló, as well as some reviews of his books. As a curious addendum, a ranking of his own articles, given by Santaló himself, is included.