The profession of the social worker, which is the subject of this book, has developed in the United States mostly within the past twenty-five years. Probably ten thousand persons are now so employed. It is known by various titles--social worker, school nurse, home and school visitor, welfare worker, hospital social worker, probation officer--varying according to the particular institution--the hospital, the court, the factory, the school--from which it has developed. But although the use of these visitors has been developed independently by each institution, and largely without consciousness of what was going on in the others, yet the same fundamental motive power has been at work in each case. Because this is so, we shall do well, at the outset of our study of home visiting, to get a clear conception of the common trunk out of which various types of home visitor have come like branches. Why has such an army of new assistants been called into existence? For this reason: In the school, in the court, in the hospital, in the factory, it has become more and more clear, in the last quarter of a century, that we are dealing with people in masses so great that the individual is lost sight of. The individual becomes reduced to a type, a case, a specimen of a class. These group features, this type of character, of course the individual possesses. He must be paid as "a hand," he must be enrolled in a school as "a pupil," admitted to the dispensary as "a patient," summoned before the court as "a prisoner." But in this necessary process of grouping there is always danger of dehumanization. There is always danger that the individual traits, which admittedly must be appreciated if we are to treat the individual according to his deserts, or to get the most out of him, will be lost sight of. We shall fail to make the necessary distinction between A and B. It is the recognition of this danger which has led, in the institutions which I have mentioned, to the institution of the social worker. Above all of her duties it is the function of the social worker to discover and to provide for those individual needs which are otherwise in danger of being lost sight of. How are these needs found? In schools, hospitals, factories, courts, and in the home visiting carried out in connection with them, one can discern the two great branches of work which in the medical sphere we call _diagnosis and treatment_.