Together with "Theory of Operator Algebras I, II", this book, written by one of the most prominent researchers in the field of operator algebras, summarises the scientific work of the author focusing on von Neumann algebras and non-commutative integration. The book is part of the recently developed part of the "Encyclopaedia of Mathematical Sciences" on operator algebras and non-commutative geometry. The book provides essential and comprehensive information to graduate students and researchers in mathematics and mathematical physics.