BackgroundActions taken to enhance and conserve the outdoor environment offer opportunities to improve the health and wellbeing of people taking part in those activities and of those who are exposed to the improved environments. Both health and environmental organisations have used conservation activities as health promotion interventions; however evidence of effect is disparate, dispersed, and limited and therefore requires novel approaches to evidence synthesis. Our aim was to systematically review the evidence and then build on the results to consider if, how, for whom, and in what contexts environmental conservation activities are beneficial to those taking part. MethodsAfter a theory-driven, mixed-method systematic review, a conceptual model was derived from the results of the primary synthesis to illustrate mechanisms of change and the role of mediators, moderators, and feedback loops that might have led to or influenced health gain. Because of the nature of the primary evidence the plausibility of key pathways was then assessed through the identification and application of further high level evidence. Since the evidence was highly dispersed, a multistage extended search strategy of academic and grey literature was used. 27 databases spanning health, social, and geographical sciences were searched for studies published in English between Jan 1, 1990, and Oct 1, 2012. 13 quantitative and 13 qualitative studies were identified. FindingsAlthough the results were largely inconclusive there was some evidence of positive effects in six of the quantitative studies to physiological, mental, and emotional health outcomes. The qualitative evidence suggested that positive experiences contribute to quality of life, particularly for marginalised groups. The benefits of these complex interventions operate through complex pathways and might differ between individual participants. The potential mechanisms, through which positive health and wellbeing benefits can occur, such as reducing social isolation and increasing levels of physical activity, are well supported by existing high level research. InterpretationConservation activities can offer a win-win situation, with potential to improve both the environment and health and wellbeing through one delivery mechanism. Although the extended review process maximised the available evidence, further robust research is needed. FundingThis research and the original systematic review were funded by the National Institute for Health Research School for Public Health Research (NIHR SPHR). The European Centre for Environment and Human Health (part of the University of Exeter Medical School) is part financed by the European Regional Development Fund Programme 2007 to 2013 and European Social Fund Convergence Programme for Cornwall and the Isles of Scilly.