Historically, qualitative and quantitative approaches have been applied relatively separately in synthesizing qualitative and quantitative evidence, respectively, in several research domains. However, mixed methods approaches are becoming increasingly popular nowadays, and practices of combining qualitative and quantitative research components at the primary empirical study level recently have increased significantly in frequency. Nonetheless, this mixing of methods is only seldom considered and adapted at the synthesis level. Thus, we presented an overview of the recent developments concerning mixing methods at the synthesis level, and explored the possible contributions and challenges of mixed methods research to the integration of qualitative and quantitative research at this level. (Contains 1 figure and 2 tables.)