Unless a subject is brought before the medical profession from time to time, it may easily slip into the background and be little thought of. Since over five hundred references to fat embolism are found in the literature, only the most striking facts concerning it will be presented in this paper. Clinically, fat embolism is difficult to substantiate conclusively. This difficulty is realty not the fault of medical practitioners but is due to the nature of the disease, which makes its definite recognition difficult. Some of the newer experimental phases of its diagnosis are presented in this paper in the hope that they may be of aid to the ultimate solution of the problem. GENERAL SURVEY Historical Observations. —As early as 1669 Lower1 had injected milk intravenously in dogs, and this was probably the beginning of the knowledge of fat embolism. Magendie,2 during the years 1821 to 1836,