This meta-analysis offers a snapshot of thirty-five years (1980-2015) of research on instructed second language acquisition (ISLA). Fifty-four empirical studies involving a total of 5,051 second language learners -- sampled from six applied linguistics journals, "Applied Linguistics, Language Learning, Language Teaching Research," "The Modern Language Journal," "Studies in Second Language Acquisition," and "TESOL Quarterly" -- were aggregated for the effects of second language (L2) instruction, yielding an overall large effect size, g = 1.06, 95 % CI = 0.84-1.29. Data were further analysed to identify factors that can modulate the efficacy of instruction. While a minor difference was detected between explicit and implicit instruction, statistically significant effects were found for modes of outcome measures, learners' onset L2 proficiency, research settings, and intensity of instruction.