Informal communication networks within firms represent the connections among workers across which information and organizational learning are transferred, status is exchanged, and social, mentoring, and administrative relationships are shared. Despite wide attention to the implications of the patterns of these relationships, there has been no large-scale comparative empirical description of informal communication networks. As a result, little is known about the natural heterogeneity across firms, and how the structure of those networks varies with attributes or performance of the organization. In this talk, I present a comparative study of high resolution, within-firm communication network structure, across a large number of organizations of varying industry, size, and formal organization. The primary meaningful variable related to informal network structure is its size, which has implications for communication and the distribution of power in firms. We also find a high level of heterogeneity unrelated to organization type, and no meaningful relationship between networks and firm performance. The scale of this heterogeneity and lack of meaningful correlations suggests that previous results based on case studies are perhaps overstated. This novel empirical perspective suggests a potential challenge and opportunity for organizational theory.