Man is an animal and, as such, is subject to the same laws of existence and the same conditions of living, as are other animals. As a part of the great organic world, he is not a being that is set apart from the ordinary and regular conditions of life, governed by different laws and requirements of existence. It is not surprising, therefore, that we find man not only able to fast for prolonged periods and able to do so with benefit, but also find him fasting under a wide variety of circumstances and for a wide variety of purposes. In the following pages, we shall briefly review the most important of the conditions under which man fasts and the purposes for which he fasts.