Abstract: The primary objective of this multicenter, multinational, epidemiological study is the identification of infectious agents, dietary factors, or other environmental exposures that are associated with increased risk of autoimmunity and type 1 diabetes mellitus (T1DM). Factors affecting specific phenotypic manifestations such as early age of onset or rate of progression or with protection from the development of T1DM will also be identified. The Environmental Determinants of Diabetes in the Young (TEDDY) is an observational cohort study in which newborns who are younger than 4 months and have high-risk human leukocyte antigen alleles in the general population or are first-degree relatives (FDRs) of patients affected with T1DM will be enrolled. Six clinical centers in the USA and Europe will screen 361 588 newborns, of which it is anticipated that 17 804 will be eligible for enrollment with just over 7800 followed. Recruitment will occur over 5 yr, with children being followed to the age of 15 yr. Identification of such factors will lead to a better understanding of disease pathogenesis and result in new strategies to prevent, delay, or reverse T1DM.