Coalitions are increasingly utilized to promote positive community health outcomes. Typically, coalitions produce more desirable outcomes when coalition functioning is strong and the coalition model is implemented with fidelity. Although theory indicates that coalitions proceed through predictable stages of development, minimal research explicitly examines functioning and fidelity at these different stages. Within a larger evaluation of Washington State Community Prevention and Wellness Initiative, this cross-sectional study employs qualitative comparative analysis to illuminate the coalition functioning conditions necessary and sufficient to produce high model fidelity at different stages of development in 43 substance misuse prevention coalitions in one state. In the formation stage, only the presence of high levels of coalition leadership was sufficient to produce high model fidelity. In the maintenance stage, three combinations of conditions were sufficient: (1) sustainability planning if, and only if, accompanied by the absence of coalition participation costs, (2) coordinator leadership, and (3) a combination of coalition leadership and team cohesion. In the institutionalization stage, two solutions were sufficient: (1) coalition leadership if, and only if, accompanied by the absence of sustainability planning, and (2) sustainability planning if, and only if, accompanied by the absence of coordinator leadership. This study illustrates several tangible steps technical assistance providers may take to increase the likelihood of achieving model fidelity. In the formation stage, skillful and inclusive coalition leadership is important. In the maintenance stage, technical assistance should focus on reducing participant-perceived costs; increasing sustainability planning; enhancing coordinator-specific leadership; and developing team cohesion and coalition leadership. For coalitions in the institutionalization stage, coalition leadership and sustainability planning may be prime targets for technical assistance.Plain Language Summary: Community coalition approaches to addressing social problems are common and have some evidence of producing positive community outcomes. Research shows that coalitions produce more desirable outcomes when coalition functioning is strong and the coalition model is implemented as it was designed. Although theory suggests that coalitions proceed through predictable stages of development, few studies look at which supports are most needed, at each stage, to improve coalition functioning and implementation. This study aims to help answer that question by determining which aspects of coalition functioning, in which combinations, are key to strong, well-functioning coalitions at different developmental stages in a sample of prevention coalitions in Washington State focused on youth substance misuse. The results suggest that having a strong coalition leadership team is key for all coalitions, regardless of development stage. For coalitions earlier in their formation, this was especially true. For coalitions further into their development, the combination of strong coordinator leadership and strong team cohesion was particularly important. Finally, for the most well-established coalitions, if sustainability planning was weak, strong coalition team leadership was key; but if coordinator leadership was weak, strong sustainability planning was critical to offset the lack of strong leadership. The results can be used to inform the types of technical assistance provided to support coalitions at different developmental stages.