We give a microscopic derivation of the speed of light relevant for energy transport in media containing randomly distributed scatterers. A comparison is made with the concept of ``mass enhancement'' in electron transport theory. The consequences for the Thouless criterion for strong localization are discussed. Finally, quantitative results are obtained for simple scatterers, such as semiclassical oscillators and dielectric spheres. We will also introduce some heuristic approaches and discuss the validity.