Georg Simmel (1858-1918) is widely recognized as an important forerunner of the social network approach. This chapter discusses the impact of Simmel’s writings on the develop-ment of social network analysis and its relevance for contemporary research. I argue that Simmel’s work was both more influential and more systematic than has usually been acknow¬ledged. In the first part I trace Simmel’s influence on social network analysis by distingui¬shing between a general structural perspective and the adoption of concrete ideas, particularly formulated in his chapters on quantitative aspects and the “web of group affiliations”. In the second part the focus is on Simmel’s concept of forms of sociation (Formen der Vergesell¬schaftung). I argue that reference to so-called basic structural properties such as group size, time or space is key to an analytical perspective that provides a specific explanation of how relationships and networks matter. The “power of structural properties” with respect to the dynamics of social relationships is illustrated by a qualitative study on changes in personal networks following the loss of the spouse. I close with implications for research into personal networks.