Traditional thinking considered the nervous system, endocrine system and immune system to be independent of each other. However, it is now widely accepted that these systems interact through the psycho-neuro-endocrino-immune network. The nervous system affects the endocrine and immune systems by releasing neurotransmitters through the hypothalamus in the hypothalamic-pituitary portal circulation. The endocrine system affects the nervous and immune systems by secreting hormones and the immune system feeds back to the nervous and endocrine systems via cytokines. Forest therapy reduces sympathetic nervous activity, increases parasympathetic nervous activity, and regulates the balance of autonomic nerves. As a result, forest therapy decreases blood pressure and heart rate and has a relaxing effect. Forest therapy affects psychological responses via the brain and nervous system thereby decreasing the scores for anxiety, depression, anger, fatigue, and confusion, and increasing the score for vigor in the POMS test. Forest therapy acts on the endocrine system to reduce stress hormone levels such as urinary adrenaline, urinary noradrenaline, salivary cortisol, and blood cortisol levels and shows a relaxing effect. Forest therapy also acts directly and indirectly on the immune system to promote NK activity by increasing the number of NK cells and intracellular levels of anticancer proteins such as perforin, granulysin and granzymes. Taken together, forest therapy brings various effects on human health via the psycho-neuro-endocrino-immune network.