In this provocative work, David N. Stamos tackles the problem of determining exactly what a biological species is: in short, whether species are real and the nature of their reality. Although many have written on this topic, The Species Problem is the only comprehensive single-authored book on this central concern of biology. Stamos critically considers the evolution of the three major contemporary views of species: species nominalism, species as classes, and species as individuals. Finally, he develops his own solution to the species problem, a solution aimed at providing a universal species concept worthy of the Modern Synthesis. This book will be of interest to philosophers of biology and of science in general, to historians of biology, and to biologists concerned with one of the most significant (and practical) conceptual issues in their field.