The physiological principles of Spa Treatment are explained on an adaptation model with three stages of adaptive modification: 1. autonomic recovery, 2. functional adaptation, 3. trophic and plastic adaptation. The stressors are the stimuli of the physical therapy which form adaptates during the spa treatment. These adaptates are directly dependent on the quantity and specificity of the stressors. Clinical symptomatology is elucidated practically with reference to two therapeutic principles and substantiated by results from spa research. The effects and long-term success of the treatment are scientifically demonstrated from the point of view of physiological adaptation.