Urbanisation has a profound effect on both people and the environment, as levels of physical activity decline and many natural ecosystems become lost or degraded. Here we draw on emerging research to examine the role of green spaces in providing a venue for outdoor physical activity, and in enhancing the benefit of a given amount of physical activity for urban residents. We identify critical knowledge gaps, including (1) whether (and for whom) levels of physical activity increase as new green spaces are introduced or old spaces reinvigorated; (2) which characteristics of nature promote physical activity; (3) the extent to which barriers to outdoor physical activity are environmental or social; and (4) whether the benefits of physical activity and experiences of nature accrue separately or synergistically. A clear understanding of these issues will help guide effective investment in green space provision, ecological enhancement and green exercise promotion.