This book provides a critical overview of the state-of-the-art in biological systematics, and presents a broad perspective of the subject covering its history, theory and practice. The most important current theoretical issues are reviewed, with the emphasis on the species concept, the methodology of phylogenetic reconstruction and contrasting views on the relationships between phylogenetics and systematics. The book concludes with a review of the current state of taxonomy of the main groups, and a discussion of evolutionary patterns.