Generalisability is typically only briefly mentioned in discussion sections of evaluation articles, which are unhelpful in judging whether an intervention could be implemented elsewhere, with similar effects. Several tools to assess generalisability exist, but they are difficult to operationalise and are rarely used. We believe a different approach is needed. Instead of focusing on similarities (or more likely, differences) in generic population and setting characteristics, generalisability assessments should focus on understanding an intervention's mechanism of action - why or how an intervention was effective. We believe changes are needed to four types of research. First, outcome evaluations should draw on programme theory. Second, process evaluations should aim to understand interventions' mechanism of action, rather than simply 'what happened'. Third, small scoping studies should be conducted in new settings, to explore how to enact identified mechanisms. Finally, innovative synthesis methods are required, in order to identify mechanisms of action where there is a lack of existing process evaluations.