Luis Antonio Santaló (Spain 1911 - Argentina 2001) contributed to several branches of geometry, perhaps his most outstanding achievement being his establishment of the mathematical foundations of stereology and its applications. Considerable power of abstraction, a brilliant geometric intuition, and his exceptional abilities 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, 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 contributions 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, and convex geometry.