In theory, employee turnover has important consequences for groups, work units, and organizations. However, past research has not revealed consistent empirical support for a relationship between aggregate levels of turnover and performance outcomes. In this paper, we present a novel conceptualization of turnover to explain when, why, and how it affects important outcomes. We suggest that greater attention to five characteristics—leaver proficiencies, time dispersion, positional distribution, remaining member proficiencies, and newcomer proficiencies—will reveal dynamic member configurations that predictably influence productive capacity and collective performance. We describe and illustrate the five properties, explain how particular member configurations exacerbate or diminish turnover’s effects, and present a new measurement approach that captures these characteristics in a collective context and over time.