  Until very recently the physical mechanism driving oscillations in
$\beta$~Cep and other early type stars has been a mystery. The breakthrough
came with the publication of new OPAL and OP opacity data. Model calculations
with the new opacities have demonstrated that the pulsations are driven by the
familiar $\kappa$-mechanism, acting in the metal opacity bump at $T \approx 2
\times 10^5$K. The mechanism excites the low order $p$- and $g$-modes in the
upper part of the instability strip and the high order $g$-modes in the lower
part of the strip. The theoretical instability domains agree well with the
observed domains of the $\beta$~Cep and the SPB stars. In this review I present
these recent theoretical results and discuss their consequences for our
understanding of B stars pulsations.
