One of the open challenges in subnuclear physics is to understand the
non-perturbative regime of Quantum Chromodynamics, including the world
of the nucleon and its excitations. One of the key issues is to identify
the relevant degrees-of-freedom and the effective forces between them.
A necessary step towards this aim is undoubtedly a
precise knowledge of the experimental spectrum and the properties of
Recently, large progress has been made e.g. based on
photoproduction experiments providing not only differential
cross section measurements but also high quality single and
double polarization observables.
In the talk, among others results, these results will be discussed.