The purpose of this qualitative study was to investigate the perception of EFL teachers and learners on the fundamental principles and constructs of critical thinking, the main characteristics of a critical thinker, and strategies for reinforcing critical thinking ability. Semi-structured interviews with eight EFL teachers and ten learners, who were theoretically sampled from three different contexts of public, private, and seminary schools in Sabzevar and Qom, Iran, were conducted using the constructivist grounded theory methodology (Charmaz, 2008). Using the MAXQDA software version 12, the data were analyzed based on the three levels of the open, axial, and selective coding of grounded theory. A model of critical thinking was proposed with ELT teachers' and learners' awareness as the core category and the characteristics of a critical thinker and strategies for reinforcing critical thinking ability as sub-categories. The results of the study revealed several pedagogical implications for EFL teachers to help their learners develop critical thinking skills, for EFL learners to think more deeply, solve problems better, communicate and collaborate more effectively, and for curriculum developers and syllabus designers to put certain critical thinking activities in textbooks and support in-service classes for teachers.