In the review of literature on stress phenomenology and mechanisms stress is defined as a holistic adaptive psychophysiological reaction in response to a physical or emotiogenic stimulus. Physiological and psychological approaches to stress analysis are demonstrated. Stress is regarded as a nonspecific reaction to stimuli characterized by particular changes in responses of organism. Stress is a part of physiological and personality adaptation. Stress-reaction may produce equally a negative and positive effect on human health and performance.