These essays originally appeared as a special issue of the journal International Organization (59, 4, Fall 2005). The collection represents a shift in research on international institutions, from a preoccupation with showing that institutions mattered in world politics to a more recent exploration of the processes, mechanisms, and conditions under which they matter. The emphasis is resolutely eclectic. Rationalism is the social theory of choice for some contributors; others are more comfortable with social constructivism. Still others combine the two. Employing numerous theories, methods, and data sources, the essays in Part II offer fascinating, richly detailed, multi-causal stories of how NATO, the European Union, and other institutions may socialize states and individuals, thus building new senses of belonging.