This article reports the findings of a one-year longitudinal study that investigated the impact of group work on the development of students' critical thinking in Hong Kong secondary schools. It explores whether the participation of teachers in a group-based teaching intervention adapted from an earlier study conducted in the United Kingdom (UK) facilitated students' use of critical arguments in Liberal Studies lessons. In addition to examining students' critical thinking skills through test performance and the use of reasoned justifications in written class-work, the article also discusses the applicability to the Hong Kong context of the programme on which the intervention was based. In general, the results of the study indicate that group work is more effective than whole-class instruction in developing students' critical-thinking skills and that students make better progress in "teacher-supported" than 'student self-directed' group work.