CREW is an NIH-funded project consisting of 12 individual U.S. birth cohorts and two scientific centers working together to identify phenotypes and causes of childhood asthma. CREW will include data from a large number of children (over 9,000 at birth, 6,000-7,000 who are still being followed, and at least 5,667 expected to enroll in CREW) and their families, with broad diversity in terms of ethnicity, family characteristics, neighborhoods and geographic locations. One of the primary goals of CREW is to put together sets of data and samples of participating cohorts to identify phenotypes of childhood asthma (i.e. specific subtypes of asthma that can be distinguished by clinical features such as natural history, triggers, exacerbation frequency, concurrent allergies, lung function, sex, etc). As we obtain mechanistic insights about personal and early life risk factors, we will connect asthma phenotypes with underlying causes and pathogenic mechanisms to define endotypes of childhood asthma.