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East Asia is dominated by a typical monsoon climate. The East Asian summer monsoon (EASM) exhibits considerable variability on a wide range of time scales during the 20 th century. A substantial portion is the multi-decadal variability. Over the recent decades, the EASM has been weakening from the end of the 1970s which results in a “southern China flood and northern China drought” rainfall pattern. Understanding the mechanisms responsible for the weakening tendency has been a challenge for climate research community. Examinations on the long-term change of the EASM during the 20 th century find no significant trends, indicating the pronounced weakening tendency of the EASM in recent decades is unprecedented. After documenting the prominent features of the interdecadal climate transition, a review is presented in this paper on the proposed explanations to the observed changes. The proposed factors include the Indian Ocean and far western Pacific warming, the tropical central-eastern Pacific warming, the weakening sensible heat source over the Tibetan Plateau, and the aerosol forcing, as well as internal variability. While parts of the monsoon circulation changes can be explained in terms of the proposed mechanisms, it is still beyond the scope of our current knowledge to present a complete picture. Much remains to be learned about the mechanisms that produce such multi-decadal changes in the EASM, but it seems still unclear whether human activities and global warming are playing significant roles.