  This paper studies the small time behavior of the heat content of
rotationally invariant $\alpha$--stable processes, $0<\alpha \leq 2$, in
domains in $\R^d$. Unlike the asymptotics for the heat trace, the behavior of
the heat content differs depending on the ranges $0<\alpha<1$, $\alpha=1$ and
$1<\alpha\leq 2$.
