A memory module includes a plurality of memory devices and a buffer chip. The buffer chip manages the memory devices. The buffer chip includes a refresh control circuit that groups a plurality of memory cell rows of the memory devices into a plurality of groups according to a data retention time of tire memory cell rows. The buffer chip selectively refreshes each of the plurality of groups in each of a plurality of refresh time regions that are periodically repeated and applies respective refresh periods to the plurality of groups, respectively.