Green exercise refers to physical exercise undertaken in natural environments. Physical exercise is well known to provide physical and psychological health benefits. There is also good evidence that viewing, being in, and interacting with natural environments has positive effects, reducing stress and increasing the ability to cope with stress, reducing mental fatigue and improving concentration and cognitive function. The concept of Green exercise has therefore grown out of well-established areas such as the attention restoration theory within environmental psychology which have tended to focus on the psychological and physical effects of viewing nature (e.g., see the work of Kaplan and Ulrich) and well-recognised work about the psychological benefits of physical exercise. The potential role of green exercise in physical and mental health (e.g., due to nature-deficit disorder) attracted increasing attention from the early twenty-first century, particularly through the research work of Jules Pretty and Jo Barton at the University of Essex. and several funded programs (see examples). Research has involved participants from many different cohorts including adults, young people and vulnerable groups such as those with mental illness (see research). Theory Green exercise can be usefully considered from a number of theoretical perspectives including: Exercise psychology biophilia and evolutionary psychology Environmental psychology theories, such as attention restoration theory Research Adults Research examining the impact of Green Exercise in adults has demonstrated significant benefits for self-esteem and mood. The largest study of green exercise in adults involved meta-analytic methodologies to analyse the results of 10 studies based on 1252 green exercise participants. Results indicated that "the overall effect size for improved self-esteem was d = 0.46 (CI 0.34−0.59, p < 0.00001) and for mood d = 0.54 (CI 0.38−0.69, p < 0.00001). Dose responses for both intensity and duration showed large benefits from short engagements in green exercise, and then diminishing but still positive returns. The study also revealed that all types of green environment improved self-esteem and mood, however the greatest improvements were experienced from environments where water was present. Similarly all cohorts of participants experienced improvements in self-esteem and mood, however those suffering from mental illness experienced the greatest increases in self-esteem. Vulnerable groups The outcomes of the meta-analysis by Barton and Pretty indicated that green exercise may have therapeutic applications. This has led to the idea of Green Care. Green Care comprises all types of therapeutic green exercise activities including social and therapeutic horticulture, animal assisted interventions, ecoptherapy,