Bilingualism is a world-wide phenomenon, featuring a vast amount of diversity. Researchers, educators and clinicians across the world face similar challenges when it comes to estimating how bilingual an individual is. There is however no consensus at present as to how bilingualism should be measured. This project aims to bring a step-change in the measurement of bilingual language experience. It will seek to establish an optimal metric informed by an in-depth review of existing tools and a consensus among researchers, speech & language therapists and educators on what aspects of language experience to index. We aim to deliver user-friendly, online questionnaires (and their associated back-end calculators) to return measures of current and cumulative language experience in real time. The questionnaires will be available in 13 languages, and vary in length and level of detail: the shortest version will be useful when parental consultation is challenging; the longest version will yield more fine-grained measures to enable in-depth enquiries. Reliability and cross-language validity of the tools we develop will be assessed using new data from 300 children in 3 different countries. Based on this assessment, we will provide evidence-based guidance to inform users’ choice on the level of questionnaire detail most appropriate to their needs. Exploiting cutting-edge statistical techniques, we will also develop an objective method to identify early those bilingual children in need of support with their school language, helping practitioners estimate when a child who speaks a different language at home can be expected to have “caught up” with their monolingual peers. The Q-BEx project is funded by the Economic and Social Research Council from October 2019 to September 2022. See the [project webpage][1] for more detail. [1]: https://q-bex.org/