This book presents in full the work of the Italian theoretical physicist Ettore Majorana and explains its impacts, which are still being felt.  It opens with a contribution by A. Zichichi that considers in depth the scientific genius of Majorana. This introductory chapter is followed, in chronological order, by the eleven scientific papers by this great scientist, in most cases translated into English for the first time. Each paper is accompanied by a comment from an expert in the field in question. Although very few in number, Majorana's papers constitute a heritage of undeniable value and extraordinary scientific meaning, since they laid the foundations for research fields that remain topical today. With this in mind, two additional contributions on ongoing developments in these research fields are included: one on neutrino physics and the other on Majorana fermions in condensed matter. The volume closes with a note on Majorana's life until his ill-fated disappearance.