  Among A-type main-sequence variables, pulsations of $\delta$ Sct and $\gamma$
Dor variables are driven in the He II ionization zone, while H-ionization zone
and strong magnetic fields seem to play roles in the excitation of high-order
p-modes in rapidly oscillating Ap (roAp) stars. Pulsations in B-type variables,
$\beta$ Cephei and slowly pulsating B (SPB) stars are excited by the
$\kappa$-mechanism at the Fe-opacity bump at $T\approx 2\times10^5$K. In
addition, the strange-mode instability seems responsible for the excitation of
pulsations in luminous AB-supergiants ($\alpha$ Cygni variables). We discuss
excitation mechanisms for pulsations in A- and B-type variables stars.
