  We show that the minimal speed for the existence of monotonic fronts of the
equation $u_t = (u^m)_{xx} + f(u)$ with $f(0) = f(1) = 0$, $m >1$ and $f>0$ in
$(0,1)$ derives from a variational principle. The variational principle allows
to calculate, in principle, the exact speed for arbitrary $f$. The case $m=1$
when $f'(0)=0$ is included as an extension of the results.
