Biological Systematics has changed dramatically during the past 60 years from a handicraft or art to an accepted branch of science proper, due to the work of Willi Hennig, who was born in 1913. The scientific method of reconstructing phylogenetic relationships of organisms bases on Hennig’s approach, the Phylogenetic Systematics . The method is now so widely accepted and applied that it can firmly be regarded a paradigm, named ‘cladistics’. In contrast, the life and personality of its founder is remarkably little known in the scientific community. The present book offers a detailed biography of Willi Hennig, and traces the roots of his thinking from his schooldays until his death in 1976. Some outstanding academic teachers and friends of his are introduced, too. The book offers an insight into the historical development of a ‘scientific revolution’, and highlights the life and the work of a ‘cautious revolutioniser’ in a Germany of dictatorship, war, and separation.