  We investigate the pulsational properties of Pre--Main-Sequence (PMS) stars
by means of linear and nonlinear calculations. The equilibrium models were
taken from models evolved from the protostellar birthline to the ZAMS for
masses in the range 1 to 4 solar masses. The nonlinear analysis allows us to
define the instability strip of PMS stars in the HR diagram. These models are
used to constrain the internal structure of young stars and to test
evolutionary models. We compare our results with observations of the best case
of a pulsating young star, HR~5999, and we also identify possible candidates
for pulsational variability among known Herbig Ae/Be stars which are located
within or close to the instability strip boundaries.
