In the first part of this thesis an attempt has been made to provide a concise account of the history of hospital infection. Neglected early contributions, particularly from the eighteenth and nineteenth centuries, are reviewed; and the survey is continued up to 1966. It is hoped that this section, together with the subsequent discussions, may usefully supplement the review by Williams et al. (1960) which deals almost exclusively with the literature from the end of the Second World War until 1960. The investigations embodied in the second and third parts of the thesis began in 1960. During the first year of the work, a study was made of all clinically-apparent infections which developed in the maternity department and in the majority of medical and surgical wards of a general hospital. To the broad picture of hospital infection which emerged, were added the results of a systematic bacteriological examination of the hospital environment. Following this general study, a detailed enquiry was made into the mechanism of cross-infection in a dermatology department. The conditions encountered in such a department offered unique opportunities for the study of the bacteriological interaction of patients and environment. This investigation occupied a period of four years. It was mainly concerned with the dispersal of pathogenic bacteria by patients, and with the effects of specific counter-measures on both the incidence of cross infection and of bacterial contamination of the environment.