Discovering a theory of change for health promotion in small- and medium-sized enterprises highlights important lessons about how successful workplace health interventions work and the conditions conducive to positive outcomes for ‘hard to reach groups’. In the evaluation of targeted health promotion initiatives carried out by the Workwell project in Sandwell, a theory of change has emerged that indicates the need for a sensitive understanding of the contexts of interventions and the importance of developing mechanisms appropriate to local conditions and stakeholder expectations.