As sociologists look into the new century for sources of explanatory leverage, we argue that small group research contains untapped theoretical potential. Small groups have been largely ignored as a topic in their own right; instead they are treated as a “black box” in which other social phenomena are observed. We propose a reassessment. By opening the “black box,” sociologists will find that the core issues of the discipline come together in small groups. We draw together the literatures of five domains, across which the findings on small groups are fragmented. These findings show that small groups are the locus of both social control and social change, where networks are formed, culture is created, and status order is made concrete. We refer to these as the controlling, contesting, organizing, representing, and allocating features of small groups. As the crossroads where agency meets structure, small groups offer the micro foundations for a twenty-first century sociological agenda.