Hydrotherapy involves the application of water, internally or externally, to treat physical or psychological conditions. There are two main types: immersion, where the entire body is submerged, and non-immersion. Water has properties like buoyancy, viscosity and hydrostatic pressure that make it useful for wound care, pain relief, edema control and exercise. Proper assessment of each patient's needs and risks is required to safely apply hydrotherapy techniques like whirlpools, contrast baths or non-immersion devices.