  The theory of heavy ion double charge exchange (DCE) reactions $A(Z,N)\to
A(Z\pm 2,N\mp 2)$ is recapitulated emphasizing the role of Double Single Charge
Exchange (DSCE) and pion-nucleon Majorana DCE (MDCE) reactions. DSCE reactions
are of second--order distorted wave character, mediated by isovector
nucleon-nucleon (NN) interactions. The DSCE response functions resemble the
nuclear matrix elements (NME) of $2\nu 2\beta$ decay. The MDCE process proceeds
by a dynamically generated effective rank-2 isotensor interaction, defined by
off--shell pion--nucleon DCE scattering. In closure approximation pion
potentials and two--nucleon correlations are obtained, similar to the neutrino
potentials and the intranuclear exchange of Majorana neutrinos in $0\nu 2
\beta$ Majorana double beta decay (MDBD).
