Research has shown that formative assessment can have large positive effects on student achievement, but the effects vary substantially and are not always positive. In addition, the underlying mechanisms responsible for the learning effects are not well understood. We present a framework developed to support research efforts to identify these mechanisms. The framework includes formative assessment processes as well as students’ learning processes, defines and exemplifies the term mechanisms in relevant contexts, indicates possible mechanisms, and identifies aspects that require further research to characterise and explain the mechanisms through which formative assessment affects student achievement. Finally, we exemplify how the framework may be used to design studies capable of providing the robust evidence required for drawing conclusions about the mechanisms.