Are species worth saving? Can they be resurrected by technology? What is the use of species in biomedicine? These questions all depend on a clear definition of the concept of 'species', yet biologists have long struggled to define this term. In this accessible book, John S. Wilkins provides an introduction to the concept of 'species' in biology, philosophy, ethics, policymaking and conservation. Using clear language and easy-to-understand examples throughout, the book provides a history of species and why we use them. It encourages readers to appreciate the philosophical depth of the concept as well as its connections to logic and science. For any interested reader, this short text highlights the complexities of a single idea in biology, the problems with the concept of 'species' and the benefits of it in helping us to answer the bigger questions and understand our living world.