Niklas Luhmann (1927-1998) ranks as one of the most important German social theorists of the twentieth century. His works have been highly influential in sociology and other social sciences, including organization studies. The significance of organizations for Luhmann can be traced in his biography: at the beginning of his career, he spent almost eight years as a legal expert in public administration, through which he gained professional expertise in the function of organizations. This practice inspired much of his later theoretical work at Harvard's Graduate School for Public Administration, the University for Public Administration at Speyer, the Center for Social Research in Dortmund, where he was head of department, and at the Department for Sociology at Bielefeld University, where he largely refrained from administrative work. This book, which was one of Luhmann's last, can be regarded as his conclusion to over thiry years of research on organized social systems.