  This paper studies the phenomenon of invasion for heterogeneous
reaction-diffusion equations in periodic domains with monostable and combustion
reaction terms. We give an answer to a question rised by Berestycki, Hamel and
Nadirashvili in [5] concerning the connection between the speed of invasion and
the speed of fronts. To do so, we extend the classical Freidlin-Gartner formula
to such equations, using a geometrical argument devised by Rossi in [17], and
derive some bounds on the speed of fronts using estimates on the heat kernel.
