The medical profession in the United States owes a debt to Dr. Simon Baruch, of Columbia University, for advancing greatly the practice and teaching of hydrotherapy, and the author is under personal obligations to him for many sug gestions. While the author is a firm believer in using physiologic therapeutics wherever possible, he by no means wishes to exclude the use of drugs. Rational therapeutics calls for their use, but reference to them is necessarily omitted here. It is also true that many diseases are amenable to hydrotherapeutic treat ment whose names are not mentioned, but the underlying principles are stated, and it remains for the practitioner to apply them, along with other remedies, for the improvement of the circulation, for enhancing the tone of the skin, for an effect upon the nervous system, or in any other way that moisture or heat or cold may be deemed useful.