Hot or cold water administered externally or internally may be effective in treating conditions ranging from stress and pain to toxins, bacteria, and viruses that cause disease. In theory, hot water soothes and relaxes the body, while cold water discourages inflammation. Contrast therapies, those that alternate between hot and cold water, stimulate circulation. The most frequent clinical uses of hydrotherapy include whirlpool baths to rehabilitate injuries and alleviate stress, fever induction through heat stress, and neutral baths for relaxation.

Despite its numerous clinical applications, hydrotherapy holds promise as an inexpensive means of preventing and treating many common health conditions in the privacy of the home. Most hydrotherapy techniques can easily be performed, for example relieving minor trauma through the application of ice or soothing a rash with a cornstarch bath. Physiological effects of hydrotherapy have been studied, but most clinical (therapeutic) effects have not.