This is a new protocol for a Cochrane Review (intervention). The primary objective is to examine the immediate and long-term effects of second language (L2) vocabulary interventions targeting L2 learners up to six years of age on vocabulary and social-emotional well-being. The secondary objectives are to examine associations between L2 vocabulary interventions and general characteristics of L2 learners (e.g. age, L2 exposure and L1 skills), as well as specific characteristics of L2 learners who do not appear to benefit from treatment. Read it here on the Cochrane Library.