Evolving Constants of Motion

A critical presentation of Rovelli's ``evolving constants of motion'' is given. Previous criticisms by Kucha\v{r} concerning the role of factor ordering and the non-existence of observables are dealt with and shown to be unfounded. Kucha\v{r}'s criticisms that this approach does not solve the global, multiple choice or Hilbert space problems of time are confirmed, and new insight into why this is so is obtained.