In this self-contained monograph, finite generalized quadrangles are classified by symmetry, generalizing the celebrated Lenz--Barlotti classification for projective planes. It serves as an introduction to the combinatorial, geometrical, and group-theoretical concepts that arise in the classification and the general theory of finite generalized quadrangles. Several long-standing open problems are solved by using a mixture of geometrical, combinatorial, and group-theoretical arguments. Further, a conjectural classification program for all finite transaltion generalized quadrangles is decribed. The book will be useful to advanced graduate students and researchers in geometry, group theory, or algebraic combinatorics.