In recent years there has been a surge in illegal wildlife trade. Poaching of elephants and rhinos for ivory and horn has attracted the most attention, but the trade extends to many other species and commodities. This crisis has attracted funding worth hundreds of millions of dollars, mostly directed at increased law enforcement. Practitioners and policymakers increasingly recognise the need to engage communities that live alongside wildlife as key partners in the fight. Yet there is no straightforward and widely replicable model for effective community engagement — different approaches work or fail in different situations. Here, we present an evolving Theory of Change to help practitioners and policymakers think through what might work well — and why — under different conditions.