Phenomenological relativistic quantum mechanics of theNNπsystem

The theoretical basis is given for a class of mathematically well defined $\mathrm{NN}\pi$ models. They are designed to describe nucleon-nucleon scattering, pion-nucleon scattering, pion-deuteron scattering, pion production, and absorption. The elementary degrees of freedom of the models are the nucleon, the $\Delta$ isobar, and the pion. The models are relativistic and do not require renormalization.