This is a tribute of the Accademia Nazionale dei Lincei to its member Enrico Fermi on the occasion of the 110th anniversary of his birthday. This tribute consists of three parts. It begins with a short introduction on the aspects of his extraordinary scientific activity still directly influential on fundamental and applied science. The other two parts are the translation in English, carried out by one of us (G.G.), of two fundamental Fermi’s papers, which are emblematic of these aspects. These two papers were originally written in Italian and published in Italian journals in 1927 and 1934, respectively.