This article serves as the introduction to this special issue of "Language Teaching Research" on content-based language teaching (CBLT). The article first provides an illustrative overview of the myriad contexts in which CBLT has been implemented and then homes in on the five studies comprising the special issue, each conducted in a distinct instructional setting: two-way Spanish-English immersion in the USA, English-medium "nature and society" lessons taught at a middle school in China, English-medium math and science classes in Malaysian high schools, English-medium history classes in high schools in Spain, and "sheltered instruction" classes for English language learners in US schools. In spite of such divergent contexts, the five studies converge to underscore the pivotal role played by teachers in CBLT and the concomitant need for professional development to support them in meeting some of the challenges specific to CBLT. (Contains 1 figure.)