The book contains a selection of 43 scientific papers of the great mathematician Ennio De Giorgi. All papers are written in English and 17 of them appear also in their original Italian version. The editors provide also a short biography of Ennio De Giorgi and a detailed account of his scientific achievements, ranging from his seminal paper on the solution of Hilbert's 19th problem to the theory of perimeter and minimal surfaces, the theory of G-convergence and the Foundations of Mathematics.