A century after his birth, Ettore Majorana is rightfully considered one of the greatest physicists of the first half of the last century. With this volume the Italian Physical Society presents a collection of Ettore Majorana's scientific papers 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.

Scientific Paper no. 1a.- On the splitting of the Roentgen and optical terms caused by the electron rotation and on the intensity of the cesium lines.- Scientific Paper no. 1b.- Majorana dr Ettore: Search for a general expression of Rydberg corrections, valid for neutral atoms or positive ions.- Scientific Paper no. 2.- On the formation of molecular helium ion.- Scientific Paper no. 3.- On the possible anomalous terms of helium.- Scientific Paper no. 4.- Pseudopolar reaction of hydrogen atoms.- Scientific Paper no. 5.- Theory of the incomplete P? triplets.- Scientific Paper no. 6.- Oriented atoms in a variable magnetic field.- Scientific Paper no. 7.- Relativistic theory of particles with arbitrary intrinsic angular momentum.- Scientific Paper no. 8.- On nuclear theory.- Scientific Paper no. 9.- A symmetric theory of electrons and positrons.- Scientific Paper no. 10.- The value of statistical laws in physics and social sciences.- The Inaugural Lecture.- Preliminary notes for the inaugural lecture.