The complex idea of 'species' has evolved over time, yet its meaning is far from resolved. This comprehensive work takes a fresh look at an idea central to the field of biology by tracing its history from antiquity to today. John S. Wilkins explores the essentialist view, a staple of logic from Plato and Aristotle through the Middle Ages to fairly recent times, and considers the idea of species in natural history - a concept often connected to reproduction. Tracing 'generative conceptions' of species back through Darwin to Epicurus, Wilkins provides a new perspective on the relationship between philosophical and biological approaches to this concept. He also reviews the array of current definitions. "Species" is a benchmark exploration and clarification of a concept fundamental to the past, present, and future of the natural sciences.