This series of papers, all of which have been published previously in periodical form, is the outcome of a house-to-house investigation of infant mortality in four wards of Boston made in 1910-11 and 1911-12 by the Research Department of the Boston School for Social Workers under a grant from the Russell Sage Foundation. During the second year the writer was in charge of the field work. He was later commissioned to write the report of the investigation for publication. In undertaking this task it appeared that no particular service would be rendered by publishing another study of the subject which dealt only with local conditions and which was prepared with only incidental reference to the data gathered elsewhere in other investigations. It did appear, on the other hand, that there was a place for a book dealing with the problem of infant mortality in the United States and based on all the studies of the subject that are available. A number of such investigations have recently been made in different parts of the United States but, so far, the results of these have never been brought together and compared with a view to finding the common relationships and laws involved. This series of papers is an attempt to meet this need. The sources of the study, therefore, are very numerous. Since, however, they are cited in footnotes in the various chapters they need not be mentioned here. In view of the prominence given to previously unpublished data, gathered in the house-to-house investigation in four wards of Boston, it will be necessary to describe briefly the scope and method of this investigation. The four wards were 6, 8, 13, and 17, the boundaries of which correspond roughly with the sections commonly known as the North End, the West End (both of these lying in the oldest parts of the city), South Boston (lower half), and Roxbury (in part). The investigation was based on the calendar year 1910. Transcripts of the original records of all the births and infant iii