The aim of the current systematic review and meta-analysis is to comprehensively synthesize the effectiveness of language promotion interventions in early childhood education and care (ECEC) settings on L1 and L2 development of dual language learners (DLLs). We will use the PICOC-Strategy (Population, Intervention, Comparison, Outcome, Context) suggested by Petticrew et al. to frame our research questions. Specifically, this review has the following research questions: (1) To which extent do language interventions accomplished in ECEC settings effect the L2 development (in the society language) of DLLs? (2) To which extent do bilingual language interventions (with L1 as language of instruction) accomplished in ECEC effect the L1 development (in the family language) of DLLs? (3) Are there significant differences in the effectiveness of different language interventions (additive vs. integrated vs. bilingual/two-way-immersion) to promote the L2 development of DLLs? (4) Are language interventions accomplished in ECEC settings more effective in supporting the L2 development (in the society language) of DLLs, when they start early in life (before the age of three)? (5) Are language interventions more effective in supporting the L2 development (in the society language) of DLLs when they are implemented with high fidelity/high quality? (6) Are language interventions with teachers as implementers more effective in supporting the L2 development (in the society language) of DLLs than language interventions with external implementers? (7) Are language interventions with higher intervention dosage (exposure x attention) more effective in supporting the L2 development (in the society language) of DLLs?