I guess it depends on how cool it is in bubs room as to whether they are warm enough under or over the bag.... I know the grobag recommendation is not to use anything else over it however we found when she was under about 5 mths (pre-rolling) and in the mid-winter (2-4 months old) that we kept her room at 16 and sometimes it was cooler so even if we rugged her up underneath we didn't feel that she was warm enough so we chose to put blankets and sheets over her. Now that she is rolling, standing, crawling we will just have to use HEAPS of layers underneath the bag.