Recent studies in collaborative management identify social and communal trust as a key determinant of positive socio-ecological outcomes. Social trust in turn derives from fair and equitable forms of representation, participation, and revenue distribution. While many recent studies have provided in-depth cases on how formally constituted rules and procedures mediate social trust in the governance of natural resources, there is a need for more research on the role of informal institutions – social norms that are enforceable but not fully codified – in enhancing or derailing inter-communal trust, thereby crucially determining ecological and social outcomes. In this chapter, we examine – based on comparative analysis of co-management schemes from Eastern and Southern Africa – how informal institutions (mainly customary authorities) contribute to intra-communal trust. Specifically, we are interested in how the integration of informal institutions in the form of customary authorities—de facto institutions governing among others historical claims to collective rights to, and adjudicating “tradeoff conflicts” over wildlife – is crucial to success of collaborative management. The chapter potentially contributes to enhancing our theoretical understanding of how intra-communal trust along with institutional integration co-determines resource and ecological outcomes, and it does so with “empirical evidence” drawn from “multiple cases from multiple countries.”