  The progress made in understanding spontaneous toroidal rotation reversals in
tokamaks is reviewed and current ideas to solve this ten-year-old puzzle are
explored. The paper includes a summarial synthesis of the experimental
observations in AUG, C-Mod, KSTAR, MAST and TCV tokamaks, reasons why turbulent
momentum transport is thought to be responsible for the reversals, a review of
the theory of turbulent momentum transport and suggestions for future
investigations.
