Phylogenetic systematics comprise the principles and methods by which we reconstruct the evolutionary history (phylogeny) of organisms and transform this reconstruction into a biological classification of these organisms. The most important progress in designing the tools for phylogenetic reconstruction was initiated by the German entomologist Willi Hennig (1913-1976), who clarified or redefined the goals of phylogenetic systematics in a book published in 1950: Grundzüge einer Theorie der phylogenetischen Systematik. An extensively revised, English translation was published in 1966: Phylogenetic Systematics. W. Hennig's "phylogenetic systematics" undoubtedly was a very significant contribution to systematics, by some systematists and philosophers even characterized as a "revolution". Hennig's redefinition and clarification of the concepts of monophyly and phylogenetic relationships created a sound foundation for systematics in general. After decades of focussing on species-level problems, Hennig redirected the interest of systematists towards the study of higher taxa and the reconstruction of phylogenetic relationships between them. A phylogenetic system is now almost universally accepted as the most useful general reference system for biology. It has been able to accommodate new developments in systematics (such as quantitative cladistics and molecular systematics), evolutionary biology (such as ecological phylogenetics), and historical biogeography.