Decoherence is a quantum-mechanical process that dynamically describes the apparent loss of quantum coherence due to coupling of the system un- der observation to other degrees of freedom, which escape direct observation. Typical examples are given by scattering processes, in which the off-scattered particles (and/or radiation) are not detected. In such processes quantum correlations between the observed system and its environment become delo- calized in an effectively irreversible manner. Such quantum correlations can neither be seen by observations on one or the other system alone, nor inter- preted as statistical correlations between existing (i.e., defined, but possibly unknown) states of local systems. They truly reflect the non-local nature of quantum mechanics.