This commentary takes up conceptual issues raised in the introduction. Discussing the contributions to this volume, we offer three points. First, the concept of fragmentation needs theoretical clarification, which can be provided to some extent by sociological differentiation theory. We suggest a typology of different types of fragmentation. Second, differentiation theory helps to improve understanding of the different causes of fragmentation. Third, a high level of institutional differentiation is an important characteristic of modernity at many levels of politics. It is not fragmentation per se, but rather the (lack of) coordination of fragmented or differentiated institutions, that is a problem for global governance.