The current trend of speakers of English as a second or an additional language (English language learners) outnumbering speakers of English as a first language (native English speakers) has shifted the focus of English language teaching from the nativeness principle to the intelligibility principle. Following the intelligibility principle, this review examined the effectiveness of interventions on the intelligibility of English language learners (ELLs) and the comprehensibility of their speech by native speakers in two related but independent meta-analytic studies. Study 1 focused on intervention studies for ELLs to improve intelligibility in their speech. Robust variance estimation (RVE) generated significant effect sizes of 0.62 (p = 0.00) from 33 effect sizes in 18 independent studies. Study 2 focused on interventions for L1 native English speakers to improve their comprehensibility of ELL speech. RVE generated a statistically significant effect size of 0.24 (p = 0.04) with 20 effect sizes in 10 independent studies. Moderating analyses revealed that the measures of intelligibility and comprehensibility, and the speech task type, were significant factors explaining the effect size variations between the included studies in both analytic reviews. However, the length of scales did not significantly differentiate the effectiveness of interventions. The effects of pronunciation instruction are not sensitive to research setting, and interventions aiming to change native English speakers' (NESs') attitudes towards ELLs' accented English rather than those aiming to improve NESs' familiarity with ELLs' accented English have statistically significant effects. The current analysis generated unique and important implications for future educational practice and research on intelligible and comprehensible communication between L1 speakers and language learners beyond English.