In recent years, there has been an increasing focus on teachers’ conceptions within both mainstream and language education (e.g., Barnes et al., The handbook of research on teachers’ beliefs, Routledge, 2015; Basturkmen, System 40: 282–295, 2012; Borg, Second handbook of English language teaching, Springer, 2019; Couper, TESOL Quarterly 51:820–843, 2017; Freeman, Language Teaching 35: 1–13, 2002; Ha & Murray, Language Teaching Research 27:137–167, 2023; Kartchava et al., Language Teaching Research 24:220–249, 2020; Vogt et al., Language Assessment Quarterly 17:386–409, 2020; Wu et al., Interpreter and Translator Trainer 13:233–254, 2019). Against this backdrop, the chapter begins by tracing the origins of research on teacher conceptions. It then explores the definition of teacher conceptions, which is often used interchangeably with related terms such as teacher cognition and teacher beliefs. The chapter further examines the significance of studying teacher conceptions, with an attempt to address the necessity of investigating L2 teachers’ conceptions of CBA. Additionally, it explores the interplay between teacher conceptions and classroom practice, as well as the sources of teacher conceptions. Finally, the chapter highlights key findings from research on L2 teachers’ conceptions of CBA and identifies key gaps in the current literature.