Does it make any sense to talk about a Delta-isobar?
  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. 
F. Kleefeld (CFIF, IST, Lisbon, Portugal)