The observation of the lowest excited state of the nucleon, the Δ-resonance, by Fermi and collaborators1 in 1952–55 was the first experimental evidence for an internal structure of the nucleon. It took another 15 years until Friedman, Kendall and Taylor2 discovered quarks and gluons as constituents of the nucleon in their pioneering experiments on deeply inelastic electron scattering. As any composite system the nucleon can be excited to a rich excitation energy spectrum. The excited states of the nucleon can be characterized by their mass, spin J, parity P, and isospin I. Fig. 1 shows the excitation spectrum up to a total mass of 1.7 GeV/c2. The excited states of the nucleon decay by the strong interaction into a nucleon in its ground state and one or several mesons with lifetimes of about 5 · 10−24 s, corresponding to a typical width of roughly 100 MeV. Because of their large widths the excited states are often called resonances. While all excited states have a strong decay branch into the pion + nucleon channel, only the S11(1535) resonance (J=1/2, I=1/2) with a mass of 1535 MeV is strongly coupled to the η meson. The observation of an η meson is thus a selective signal for the excitation of the S11(1535) resonance.