Humans have enjoyed forest environments for ages because of the quiet atmosphere, beautiful scenery, mild climate, pleasant aromas, and fresh, clean air. In Japan, since 2004, serial studies have been conducted to investigate the effects of forest environments on human health. We have established a new medical science called Forest Medicine. The Forest Medicine is a new interdisciplinary science, belonging to the categories of alternative medicine, environmental medicine and preventive medicine, which encompasses the effects of forest environments on human health. It has been reported that forest environments have the following beneficial effects on human health:1. Increase human natural killer (NK) activity, the number of NK cells, and the intracellular levels of anti-cancer proteins, suggesting a preventive effect on cancers.2. Reduce blood pressure, heart rate, and stress hormones, such as urinary adrenaline and noradrenaline and salivary cortisol.3. Increase the activity of parasympathetic nerves and reduce the activity of sympathetic nerves.4. Increase the levels of serum adiponectin and dehydroepiandrosterone sulfate.5. In the Profile of Mood States (POMS) test, reduce the scores for anxiety, depression, anger, fatigue, and confusion, and increase the score for vigor, showing psychological effects as well.These findings suggest that forest environments may have preventive effects on lifestyle-related diseases.