With this volume the Italian Physical Society presents a collection of Ettore Majorana's scientific papers (note scientifiche) in the original language and, for the first time - with three exceptions - translated into English. Each paper is then followed by a comment (in English) of an expert in the scientific field. Contributors to the comments are Ennio Arimondo, Nicola Cabibbo, Massimo Inguscio, Luciano Maiani, Rosario Nunzio Mantegna, Francesco Minardi, Luigi Radicati di Brozolo and Antonio Sasso. A century after his birth Ettore Majorana is rightfully considered one of the greatest physicists of the first half of the last century. This volume will be of interest to the specialists of the History of Science and to the physicists concerned with problems related to Majorana's contributions.