Purpose: To develop a reliable and valid questionnaire of bilingual language status with predictable relationships between self-reported and behavioral measures. Method: In Study 1, the internal validity of the Language Experience and Proficiency Questionnaire (LEAP-Q) was established on the basis of self-reported data from 52 multilingual adult participants. In Study 2, criterion-based validity was established on the basis of standardized language tests and self-reported measures from 50 adult Spanish-English bilinguals. Reliability and validity of the questionnaire were established on healthy adults whose literacy levels were equivalent to that of someone with a high school education or higher. Results: Factor analyses revealed consistent factors across both studies and suggested that the LEAP-Q was internally valid. Multiple regression and correlation analyses established criterion-based validity and suggested that self-reports were reliable indicators of language performance. Self-reported reading proficiency was a more accurate predictor of first-language performance, and self-reported speaking proficiency was a more accurate predictor of second-language performance. Although global measures of self-reported proficiency were generally predictive of language ability, deriving a precise estimate of performance on a particular task required that specific aspects of language history be taken into account. Conclusion: The LEAP-Q is a valid, reliable, and efficient tool for assessing the language profiles of multilingual, neurologically intact adult populations in research settings.