Willi Hennig (1913-76), founder of phylogenetic systematics, revolutionised our understanding of the relationships among species and their natural classification. An expert on Diptera and fossil insects, Hennig's ideas were applicable to all organisms. He wrote about the science of taxonomy or systematics, refining and promoting discussion of the precise meaning of the term 'relationship', the nature of systematic evidence, and how those matters impinge on a precise understanding of monophyly, paraphyly, and polyphyly. Hennig's contributions are relevant today and are a platform for the future. This book focuses on the intellectual aspects of Hennig's work and gives dimension to the future of the subject in relation to Hennig's foundational contributions to the field of phylogenetic systematics. Suitable for graduate students and academic researchers, this book will also appeal to philosophers and historians interested in the legacy of Willi Hennig.