  Period changes of Delta Scuti stars have been collected or redetermined from
the available observations and are compared with values computed from
evolutionary models with and without convective core overshooting. For the
radial pulsators of Pop. I, the observations indicate (1/P) dP/dt values around
10^-7 year^-1 with equal distribution between period increases and decreases.
The evolutionary models, on the other hand, predict that the vast majority
should show increasing periods. This increase should be a factor of about ten
times smaller than observed. For nonradial Delta Scuti pulsators of Pop. I, the
discrepancies are even larger. The behavior suggests that for these relatively
unevolved stars the rate of evolution cannot be deduced from the period
changes. The period changes of most Pop. II Delta Scuti (SX Phe) stars are
characterized by sudden jumps of the order of Delta P/P approx. 10^-6. However,
at least one star, BL Cam, shows a large, continuous period increase. The
variety of observed behavior also seems to exclude an evolutionary origin of
the changes. Model calculations show that the evolutionary period changes of
pre-MS Delta Scuti stars are a factor of 10 to 100 larger than those of MS
stars. Detailed studies of selected pre-MS Delta Scuti stars are suggested.
