The Environmental influences on Child Health Outcomes (ECHO)-wide Cohort Study (EWC), a collaborative research design comprising 69 cohorts in 31 consortia, was funded by the National Institutes of Health (NIH) in 2016 to improve children's health in the United States. The EWC harmonizes extant data and collects new data using a standardized protocol, the ECHO-wide Cohort data Collection Protocol (EWCP). EWCP visits occur at least once per life stage, but the frequency and timing of the visits vary across cohorts. As of March 4, 2022, the EWC cohorts contributed data from 60,553 children and consented 29,622 children for new EWCP data and biospecimen collection. The median (interquartile range) age of EWCP-enrolled children was 7.5 years (3.7-11.1). Surveys, interviews, standardized examinations, laboratory analyses, and medical record abstraction are used to obtain information in five main outcome areas: pre-, peri-, and post-natal outcomes; neurodevelopment; obesity; airways; and positive health. Exposures include place- (e.g., air pollution, neighborhood socioeconomic status), family- (e.g., parental mental health), and individual-level (e.g., diet, genomics) factors.