ABSTRACT This study explored teachers’ perceptions of learner engagement in L2 task-based interaction. Fifty-four pre- and in-service ESL/EFL teachers with different L1 backgrounds and L2 teaching experiences were asked to define and rate learner engagement in two learner-learner interactions. The results revealed three major indicators that the teachers relied on in order to evaluate learner engagement. They included learners’ level of deep thinking and attention to their partner’s ideas, amount of content production, and level of interactiveness (e.g. amount of interaction and assistance). Only a small number of participants relied on learners’ enthusiasm and positive attitudes in interaction, perceived as emotional engagement, to judge the learner engagement level. The results are discussed in terms of similarities and differences between L2 teachers and researchers in conceptualising the construct of learner engagement. The study also provides pedagogical implications regarding assessing learner engagement when implementing classroom peer interaction and suggests guidelines for promoting teachers’ generation of L2 theories in teacher education courses.