A century after his birth, the ?gure of Ettore Majorana still arouses deep feelings in all those who studied his scienti?c work or became acquainted with his personality and appreciated his sensibility and human qualities. In this occasion the Italian Physical Society presents a collection of Majorana’s s- enti?c papers (note scienti?che) in the original language and, for the ?rst time— with three exceptions— translated into English. Each paper is then followed by a comment of an expert in the speci?c ?eld. The reason for this not common undertaking stands in the scienti?c and human ch- acter of Ettore Majorana, who can be rightfully considered one of the greatest physicists of the ?rst half of the last century. Enrico Fermi himself had so good an opinion of Majorana that, in a private conversation, he referred to him as a real genius at the same level as Galilei and Newton, such that he could most probably have been, if he only had had a longer life to dedicate to physics. Inthepresentvolumethescienti?cnotesarepresentedinchronologicalorderfollowing the numbering given by E. Amaldi in his book “La vita e le opere di Ettore Majorana”, published by Accademia Nazionale dei Lincei, Rome, 1966.