This paper evaluates the effectiveness of group cognitive behaviour therapy (GCBT) as an intervention for unipolar depressive disorders. PsychINFO and PubMed databases were selected to generate the 34 papers used for this review. Our results showed that effect sizes for GCBT over the control conditions range from small (0.1) to large (2.87) with the mean effect size of 1.10. The pre-post treatment effect sizes for GCBT range from 0.30 to 3.72 with a mean of 1.30. Convergent evidence was demonstrated across different outcome measures of GCBT. Our findings indicated that GCBT yielded outcomes better than no-treatment controls and was comparable with other treatments (including both bona fide and non-bona fide comparison treatments). It was concluded that GCBT was effective for the treatment of Unipolar depression and thus can be used with confidence. There is now an urgent need to develop and evaluate a coherent GCBT theory, in particular the roles of group processes in GCBT, before further major advancement in this area can be made.