Every year thousands of children migrate to the UK from countries around the world. This complex group varies considerably, in terms of age of arrival, reasons for migration, educational experience, and their heritage language. A large proportion of new arrivals start school in the UK with limited previous educational experience and knowledge of English. Whilst many children who have English as an additional language do very well in school, many new arrival children do not reach English language levels required to fully access the curriculum (Strand & Lindorff, 2020). In turn, they are at risk of underachieving academically, and display the largest and persistent attainment gaps in the primary school years (PISA, 2018). Whilst there is growing recognition that we must provide tailored language support for this group, there is limited research on the language acquisition trajectories of new arrival children, and their heritage languages are often hugely understudied. As such, it is unclear how we should tailor our support to target the linguistic needs of new arrivals, to ensure they have the best possible chances in school. In collaboration with the Tower Hamlets Education Partnership (THEP), this project will track the early stages of speech and language acquisition in primary-school-aged new arrival children in the London Borough of Tower Hamlets, UK. This study will take a cross-linguistic approach to understand how the linguistic structure of the heritage language (L1) and age of acquisition influences new arrivals’ acquisition of English (L2). The project will adopt a longitudinal design to track the earliest stages of L2 speech and language acquisition, and the changes that occur with increased experience in the L2. This project will advance our understanding of the fine-grained stages of language acquisition in new arrival children, providing vital foundational research needed to inform educational practice and policy.