Little is known about how practices reorganize when transitioning from traditional practice organization to team-based care. We compared practice-level (1) configuration as well as practice- and team-level (2) size and (3) composition, before and after establishing teams. We employed a pre-/poststudy using personnel lists of 1571 to 1711 staff (eg, job licenses, titles, and team assignment) and practice manager surveys. All personnel (physician and nonphysician) worked within 18 Massachusetts academic primary care practices participating in a 2-year learning collaborative aimed at establishing team-based care. We found that establishing team-based care can involve changing practice configurations and composition without substantially changing practice size.