The always distant and regarded as aloof Gustav Mahler (1860-1911) and the renowned Viennese musicologist Guido Adler (1855-1941) were friends for decades and frequently exchanged ideas on musical issues. This personal and professional closeness gives Adler's introduction to Mahler's life and work its special quality. Adler himself mentions in the preface that he has "gained insight into the innermost essence of Mahler." The present text was first published in 1914 and is here presented as a reprint of the first edition in book form from 1916.