Background: Lay health workers (LHWs) are used widely to deliver health services, particularly in under-resourced or hard-to-reach areas. Evidence of the effectiveness of these programmes is available. However, before policy makers can begin to implement or scale up such programmes, they also need an understanding of how and why the effects of these programmes vary across different groups and settings; the cost effectiveness of these interventions; and their sustainability. Objectives: To describe an approach to developing a deeper understanding of intervention impacts by combining reviews of effectiveness and cost-effectiveness with a review of the programme theory underlying interventions. Methods: First, we conducted a Cochrane review of the effectiveness of LHW interventions. Second, we are undertaking a review of studies of the costs and cost-effectiveness of LHW programmes. Third, we plan to conduct a ‘realist’ review. This review will identify the programme models underpinning LHW programmes; test the fidelity of each stage of these models; and to explore the validity of each model’s assumptions. Finally, we will attempt to combine the findings from each of these reviews using a narrative synthesis. Results: The Cochrane review of 129 trials showed that LHW interventions may be effective across a wide range of health issues including promoting breastfeeding; reducing mortality in children; and improving TB treatment outcomes. However, effect sizes across these different intervention groups were very variable and considerable heterogeneity was found within the results for some groups. Using the search strategies developed for the Cochrane review, as well as author contact and reference lists, we identified a number of cost studies of LHW programmes. We also identified process evaluations of trials and in-depth case studies of LHW programme implementation. These additional data are now being analysed and preliminary results will be presented. Conclusions: For complex interventions, our approach of combining an effectiveness review with reviews of cost-effectiveness and programme theory appears feasible. By bringing together these different data, we can provide policy makers with highly relevant information regarding the scaling up and sustainability of interventions shown to be effective in.