Purpose: The aim of this study was to develop the Language Exposure Assessment Tool (LEAT) and to examine its cross-linguistic validity, reliability, and utility. The LEAT is a computerized interview-style assessment that requests parents to estimate language exposure. The LEAT yields an automatic calculation of relative language exposure and captures qualitative aspects of early language experience. Method: Relative language exposure as reported on the LEAT and vocabulary size at 17 months of age were measured in a group of bilingual language learners with varying levels of exposure to French and English or Spanish and English. Results: The LEAT demonstrates high internal consistency and criterion validity. In addition, the LEAT's calculation of relative language exposure explains variability in vocabulary size above a single overall parent estimate. Conclusions: The LEAT is a valid and efficient tool for characterizing early language experience across cultural settings and levels of language exposure. The LEAT could be a useful tool in clinical contexts to aid in determining whether assessment and intervention should be conducted in one or more languages.