For the purposes of this review, we consider memory as any perturbation in a system, caused by external stimulation, which persists past the cessation of the initial stimulation and alters the system’s responsiveness to subsequent stimulation. Thus, memory, in the broadest of terms, is an adaptation to the past. We should point out that this definition of memory can also apply to many perturbations in non-biological systems, including viscoelastic deformation, anomalous diffusion, capacitor voltage changes, and stock market fluctuations. Gajos que não leram Proust.