This chapter provides developmental factors and second language acquisition (SLA) theory in order to provide direction for the successful creation and improved use of English language proficiency (ELP) assessments with young learners. It offers a brief account of crucial cognitive and social developments in young learners, which will help determine effective testing formats and administration choices, as well as developmentally appropriate content inclusion. The highlights some key theories of second language acquisition and discuss the implications of children's developmental factors and prior theories for the development of ELP assessments for young learners. It highlights just a few of the most influential theories with implications for assessment of young learners. Student self-assessment fits within formative assessment approaches and social cultural theory with its emphasis on student agency in their own learning. Students can be involved in characterizing their own progress and in setting goals or objectives for their language learning which can also contribute to a personalized-learning approach to instruction.