  It is shortly investigated, on what basis an experimentally observed
resonance like the Delta(1232)-isobar can be embedded into the framework of
Quantum Theory (QT), i.e. Quantum Field Theory (QFT) and Quantum Mechanics
(QM). After a short discussion of the particle concept in the context of the
"bootstrap" idea of G. Chew and S. Mandelstam we will focus on the theoretical
formalism being necessary to describe resonances in the Lagrangian or
Hamiltonian formulation of QT.
