This study examines coresident relationships in assisted living (AL) and identifies factors influencing relationships. We draw on qualitative data collected from 2008 to 2009 from three AL communities varying in size, location, and resident characteristics. Data collection methods included participant observation, and informal and formal, in-depth interviews with residents, administrators, and AL staff. Data analysis was guided by principles of grounded theory method, an iterative approach that seeks to discover core categories, processes, and patterns and link these together to construct theory. The dynamic, evolutionary nature of relationships and the individual patterns that comprise residents' overall experiences with coresidents are captured by our core category, "negotiating social careers in AL." Across facilities, relationships ranged from stranger to friend. Neighboring was a common way of relating and often involved social support, but was not universal. We offer a conceptual model explaining the multilevel factors influencing residents' relationships and social careers. Our explanatory framework reveals the dynamic and variable nature of coresident relationships and raises additional questions about social career variability, trajectories, and transitions. We discuss implications for practice including the need for useable spaces, thoughtful activity programming, and the promotion of neighboring through staff and family involvement.