The Language Experience and Proficiency Questionnaire (LEAP-Q) is a validated questionnaire tool for collecting self-reported proficiency and experience data from bilingual and multilingual speakers ages 14 to 80. It is available in over 20 languages, and can be administered in a digital, paper-and-pencil, and oral interview format. The LEAP-Q is used by researchers across various disciplines (Psychology, Neuroscience, Linguistics, Education, Communication Sciences & Disorders, etc.) to provide a comprehensive description of their bilingual participants, to substantiate a division of bilinguals into groups (e.g., early vs. late bilinguals), and to screen participants for adequate or threshold levels of language proficiency. Best practices for using the LEAP-Q include administration of the full questionnaire, consideration of acquisition and history of language use together with self-ratings of proficiency, and supplementation of self-reported data with objective language measures whenever possible. The LEAP-Q can be downloaded at no cost at https://bilingualism.northwestern.edu/leapq/.