IntroductionAnomalous diffusion processes are ubiquitous in nature [1][2][3][4][5][6]. Their occurrence is usually associated with complex systems that induce spatial and/or temporal correlations in the diffusion process. The signature of normal diffusion is the linear asymptotic dependence of the mean-square displacement of the diffusing entity (hereafter called "the particle") on time, r 2 ∼ t, t → ∞. The signature of anomalous diffusion is a nonlinear dependence on time. In particular, if the growth with time is sublinear, so that(13.1) the particle is said to be subdiffusive. (The process is superdiffusive when the limit goes to infinity.) In this chapter, we focus on the important class of subdiffusive processes for whichand where the (anomalous) diffusion exponent γ satisfies 0 < γ < 1. An interesting class of diffusive processes are so-called diffusion-limited reactions. These are processes in which diffusion is the dominant mixing mechanism and, furthermore, where the time for reactants to find one another is much longer than the time it takes for a reaction to occur following such an encounter. Therefore, in these systems diffusion is the key factor that determines the spatial distribution of reactants and the resultant reaction rate. Since diffusion is not a particularly effective mixing mechanism, diffusionlimited reactions often present extremely interesting spatial as well as temporal characteristics. In this context, it is especially appropriate to point to the pioneering work of Turing on pattern formation in reaction-diffusion systems [7]. Diffusion-limited reactions show up in a vast number of applications including not only chemical (see, e.g., [8]) but also biological (e.g., [9]), ecological (e.g., [10]) and economic processes (e.g., [11]) that have been studied over many decades.