To dogmatise on questions of medical practice is to invite controversy and tempt disaster. The highest wisdom of to-day may be completely refuted by to-morrow's discovery. Therefore, for the simple principles of disease prevention and health protection which I have put into the mouth of my Health Master, I make no claim of finality. In support of them I maintain only that they represent the progressive specialized thought of modern medical science. So far as is practicable I have avoided questions upon which there is serious difference of belief among the authorities. Where it has been necessary to touch upon these, as, for example, in the chapter on methods of isolation in contagious diseases, a question which arises sooner or later in every household, I have advocated those measures which have the support of the best rational probability and statistical support. Not only has the book been prepared in consultation with the recognized authorities on public health and preventive medicine, but every chapter has been submitted to the expert criticism of specialists upon the particular subject treated. My own ideas and theories I have advanced only in such passages as deal with the relation of the physician and of the citizen to the social and ethical phases of public health. To the large number of medical scientists, both public and private, whose generous aid and counsel have made my work possible, I gratefully acknowledge my debt.