This chapter concentrates on the asymptotic speed of propagation c₀ as an important indicator of the spread at the population level in a spatially structured population. Within the context of idealized models, it unambiguously defined the (asymptotic) speed c₀ of the spatial propagation of an infection, and characterized c₀ in terms of the basic model ingredients in such a way that the computation of c₀ from the ingredients is rather simple. Thus, we added one more indicator of the infectiousness of an agent to the list (consisting so far of R₀, r, the probability of a major outbreak, the size of a major outbreak, and the endemic level). For many ecological or agricultural systems, this is actually the most relevant indicator.