  The problem of front propagation in a stirred medium is addressed in the case
of cellular flows in three different regimes: slow reaction, fast reaction and
geometrical optics limit. It is well known that a consequence of stirring is
the enhancement of front speed with respect to the non-stirred case. By means
of numerical simulations and theoretical arguments we describe the behavior of
front speed as a function of the stirring intensity, $U$. For slow reaction,
the front propagates with a speed proportional to $U^{1/4}$, conversely for
fast reaction the front speed is proportional to $U^{3/4}$. In the geometrical
optics limit, the front speed asymptotically behaves as $U/\ln U$.
