In today's modern society, lifestyle-related diseases play an increasingly important role as the cause of bad health. Staying in and access to environments that contain natural elements such as forests, parks and gardens has been featured because of its potential to stimulating people's health and well-being. An independent research area has developed, which addresses issues that try to explain and document the health effects of access to and activities in nature. To bring together European researchers working with different aspects of the relation between nature and health, COST Action E39 "Forests, Trees, and Human Health and Wellbeing" was established in 2004 and terminated in November 2009. The main results of COST E39 was presented in the book "Forests, Trees and Human Health" published in 2011. This article focuses on three central research questions regarding the relationship between nature and health: (1) Nature's restorative and salutogenic effects on human beings, (2) Health effects of outdoor physical activities, and (3) Therapeutic interventions such as therapy gardens and forest bathing. Based on a systematic review of research carried out in the aftermath of the COST Action the following conclusions are made: (1) There is a remarkable amount of research publications showing a statistical correlation between access to nature and green areas and human physical activity and health. However, there are few studies that really prove a causal relationship, i.e. that access to green areas leads to increased physical activity and better health. (2) The existing research on outdoor activities in natural environments describes a number of positive associations and effects on health, and the evidence for some of these are convincing, albeit there must be reservations for the immediate applicability of the conclusions. (3) A lot of studies also show that nature-based therapeutic interventions work and provide clear positive effects, but there is serious criticism about the quality of the studies. Finally, the authors suggest more emphasis on research regarding the economic benefits of nature's health effects, more research on the effects of nature-based therapeutic interventions, and of broadening the theoretical basis. Furthermore, we foresee a better integration with health policies and practical implementation within urban planning and forest management.