One: General Principles; 1: Methodological Considerations; 2: Prevention of Sedentary Lifestyles: Rationale and Methods; 3: Exercise Prescription; 4: Adherence to Physical Activity; 5: Drug Therapy and Physical Activity; Two: Psychological Responses to Physical Activity; 6: Antidepressant Effects of Physical Activity; 7: Anxiolytic Effects of Physical Activity; 8: Physical Activity and Self-Esteem; 9: Overtraining and Staleness; Three: Hypothesized Mechanisms; 10: The Endorphin Hypothesis; 11: The Serotonin Hypothesis; 12: The Norepinephrine Hypothesis; 13: The Thermogenic Hypothesis; 14: Conclusion: State of the Field and Future Research