The main aim of this book is to present a systematic study of parts of the theory of von Neumann subalgebras of finite von Neumann algebras and, in particular, the theory of maximal Abelian self-adjoint subalgebras of separable II1 factors (usually called masas). The main part of the book contains a description of perturbations of von Neumann subalgebras and general results on masas, including a detailed study of singular masas in II1 factors. There is a special chapter devoted to the Pukánszky invariant of a masa. Prerequisites for reading the book are basic facts on von Neumann algebras, which are summarized at the beginning of the book. The book is suitable for graduate students wanting to learn this part of mathematics.