Given the growing multilingual student population worldwide, many countries/regions have implemented content-based instruction (CBI) programs that include content-area classrooms where language learning takes place during content learning. Scholars have suggested that more attention needs to be given to how teachers teach language in content-area classrooms and that research findings across program contexts should speak to/build on each other. This systematic review draws on sociolinguistic theories to synthesize 199 studies from 2012 –2022 about teacher understandings and practices regarding content-language integration across CBI programs worldwide, including content and language integrated learning (CLIL), English medium of instruction (EMI), immersion or dual language bilingual education (DLBE), and English as a second/additional language (ESL/EAL). Studies described uneven understandings of the content-language relationship and indicated a variety of content-language integration practices (e.g., teaching disciplinary literacy, encouraging discussion, translanguaging, writing language objectives, using visuals, and collaborating between content and language teachers). However, researchers reported challenges including lack of training, time, and curricular guidance. Based on these findings, we offer directions for research and educator preparation/professional learning to help teachers implement content-language integrated practices that support multilingual learners in developing deep content and language learning in content-area classrooms across contexts.