We present an overview of the evidence of how nature benefits mental health, popular theories used to explain the effects, and the development potential of these theories. A large body of evidence highlights the beneficial effects of nature on mental health, with observed outcomes ranging from alleviating the symptoms of psychiatric disorders to improvements in cognitive abilities. The theoretical backbone for these salutary effects of nature consists of a set of models, mainly the attention restoration theory (ART), the stress reduction theory (SRT), and the Biophilia hypothesis. However, these high-level models are only loosely related and lack a pronounced biopsychological basis. While biopsychological measurements have been used widely in recent years, these efforts have not sufficiently been reflected in theories aiming to explain the benefits of nature contact for mental health. This paper seeks to encourage interdisciplinary work and further theory development to guide both research and practice toward strategically green and healthy living conditions.