The environment during the prenatal period and in early life is a major contributor to the risk of developing childhood asthma. Birth cohort studies from single research centers have identified several factors that affect the risk for developing childhood asthma, including being exposed in early life to allergens, pollutants, viruses and bacteria, and psychosocial stress. Despite such advances, further progress in understanding the root causes of asthma have been hampered by the small size of previous studies, which makes it difficult to: 1) identify asthma risk factors with certainty, 2) know how environmental factors across the United States (U.S.) affect asthma, and 3) whether there are critical ages when pregnant mothers, infants and young children are particularly susceptible to these influences. Furthermore, different research groups tend to use different methods to study asthma, making it difficult to either compare or pool findings. One other challenge is that there are several types (i.e. phenotypes, endotypes) of childhood asthma, but these are poorly understood. To help overcome these challenges, investigators leading 12 asthma birth cohorts across the U.S. have established the Children's Respiratory Research Workgroup (CREW) consortium. CREW proposes to identify specific types of childhood asthma, develop an understanding of what early life environmental influences cause these different types of asthma and when, and identify targets for future efforts aimed at preventing childhood asthma.