There has been growing attention on the effects of forest on physiological relaxation and immune recovery, particularly in forest medicine research, from a perspective of preventive medicine. Japan is a world leader in the accumulation of scientific data on forest medicine research. In this review, we summarize the research that has been conducted in this area since 1992. We conducted field experiment, involving 420 subjects at 35 different forests throughout Japan. After sitting in natural surroundings, these subjects showed decrease in the following physiological parameters compared with those in an urban control group: 12.4% decrease in the cortisol level, 7.0% decrease in sympathetic nervous activity, 1.4% decrease in systolic blood pressure, and 5.8% decrease in heart rate. This demonstrates that stressful states can be relieved by forest therapy. In addition, it should be noted that parasympathetic nervous activity was enhanced by 55.0%, indicating a relaxed state. The results of walking experiments provided similar results. Li et al. demonstrated that immune function was enhanced by forest therapy in middle-aged employees who volunteered to participate in these experiments. Natural killer cell activity, an indicator of immune function, was enhanced by 56% on the second day and returned to normal levels. A significant increase of 23% was maintained for 1 month even after returning to urban life, clearly illustrating the preventive benefits of forest therapy. In an indoor room experiment, we conducted tests with the following: 1) olfactory stimulation using wood smell, 2) tactile stimulation using wood, and 3) auditory stimulation using forest sounds. These indoor stimulations also decreased the blood pressure and pulse rate, and induced a physiological relaxation effect. We anticipate that forest medicine will play an increasingly important role in preventive medicine in the future.