► We develop a framework which documents relationships between greenspace and health. ► The framework highlights key mediators which underpin the relationship. ► Moderators affect the relationship, such as socio-economic status and greenspace type. ► The framework can be used to inform and improve planning of research studies. A growing body of evidence investigates whether access to greenspace, such as parks and woodland, is beneficial to well-being. Potential health benefits of greenspace exposure include opportunity for activities within the space and psychological benefits of viewing and interacting with nature. However, empirical research evidence on the effects of greenspace exposure shows mixed findings. Hence we suggest that the key questions of “if, why and how?” greenspace influences health remain largely unanswered. We argue that researchers have inadequately considered the causal pathways which drive the relationship. In particular, an improved understanding is needed of potential mediators and moderators. In this paper we draw on social–ecological theories and a review of the literature to develop a novel theoretical framework which summarises current knowledge about hypothetical causal pathways between access to greenspace and health outcomes. The framework highlights how mediators – such as use of greenspace and perceptions of the living environment – drive associations between access and both physical and psychological health outcomes. We propose key moderators based on evidence that associations between greenspace and health differ by demographic factors such as gender, ethnicity and socio-economic status, living context, greenspace type and climate. We discuss the evidence for how and why these factors act as moderators and consider the implications which arise from this improved understanding of the relationship between greenspace and health. In conclusion, we discuss how the framework can be used to inform planning of research studies, and how it may be developed in the future as more evidence emerges.