There is by this point no doubt that backward, regressive, negative or degenerative movements occur in cognitive development. The question is "why?" The challenges of the phenomenon have been and continue to be mainly two: identify the range and variety of systematic backward movements that appear in development; and, provide better and better explanations for why negative movements occur and what role they play. The authors' own work on backward movement (called "reversion" in their framework) has been at much later ages (ages 10 through young adult) and at more macro levels (transitions between levels of expertise in various knowledge domains), but there is more resonance than discord between and among their findings and the ones reported in the three target articles. In their research, backward movements are systematic, predictable, and reflective of an apparently necessary phase in movement from level to level in several domains (e.g., map drawing, juggling, yoga). Their explanations for why reversion is necessary are closest to those of Gershkoff-Stowe and Thelen, but have much in common with Namy, Campbell, and Tomasello as well.