We show that close geographical proximity to mothers or mothers-in-law has a substantial positive effect on the labor supply of married women with young children. We argue that the mechanism through which proximity increases labor supply is the availability of childcare. We interpret availability broadly enough to include not only regular scheduled childcare during work hours but also an insurance aspect of proximity (e.g., a mother or mother-in-law who can provide irregular or unanticipated childcare). Using two large datasets, the National Survey of Families and Households and the public use files of the U.S. Census, we find that the predicted probability of employment and labor force participation is 4-10 percentage points higher for married women with young children living in close proximity to their mothers or their mothers-in-law compared with those living further away. (author abstract)