Some reaction-diffusion equations admit traveling wave solutions, which are simple models of a chemical reaction spreading with constant speed. Even in a heterogeneous medium, solutions to the initial value problem may develop fronts propagating with a well-defined asymptotic speed. I will describe recent progress in understanding how fronts propagate in heterogeneous media. In particular, I will discuss properties of generalized traveling waves for one-dimensional reaction-diffusion equations with variable excitation. I also will discuss multi-dimensional fronts in stationary ergodic random media.