Background: Systematic reviews of educational research require synthesis of the results from a range of studies. When these studies are diverse in nature and methodological approach, synthesis can be problematic. Purpose: To offer a critique of the process of synthesizing systematic reviews by considering the stages of the process and the problems raised at each stage. Source of evidence: A brief review of the literature on the role of systematic reviews in education is presented, together with a description of the staged process of a systematic review. Main argument: Synthesis of systematic reviews is difficult on account of the fact that they are heterogeneous across different aspects of the research process. Moreover, synthesis of such systematic reviews may be neither feasible nor desirable. Conclusions: Clear and manageable research questions are an important prerequisite for synthesis; Bayesian synthesis should be used where appropriate; and the minimization of bias must be maintained through the stages of synthesis in any research.