Julius Petersen''s paper, Die Theorie der regulären graphs in Acta Mathematica, volume 15 (1891), stands at the beginning of graph theory as we know it today.The Danish group of graph theorists decided in 1985 to mark the 150th birthday of Petersen in 1989, as well as the centennial of his paper.It was felt that the occasion called for a presentation of Petersen''s famous paper in its historical context and, in a wider sense, of Petersen''s life and work as a whole. However, the readily available information about Julius Petersen amounted to very little (not even a full bibliography existed) and virtually nothing was known about the circumstances that led him to write his famous paper.The study of Petersen''s life and work has resulted in several papers, in particular a biography, a bibliography, an annotated edition of the letters surrounding Petersen''s paper of 1891, an analysis of Petersen''s paper and an annotated edition of parts of Petersen''s correspondence with Sylow on Galois theory. The first four of these papers, together with a survey of matching theory, form the first part of this book. In addition to these five special papers, there are papers submitted in the celebration of the Petersen centennial.