This study conducts a theoretical examination of an evidence-based approach (EBA) to inquiry on language policy and language-in-education policy (LPLEP), as well as a methodological examination of the feasibility of evidence-based LPLEP inquiry based on actual studies. It argues that the EBA’s core notions are wholly applicable to applied linguistics and LPLEP inquiry. Data stem from a primary school English language teaching programme implemented in Japan since the 2000s.