  The proton mass arises from spontaneous breaking of chiral symmetry and the
formation of constituent quarks. Their dynamics cannot be tested by proton
tomography but only by studying excited baryons. However, the number of excited
baryons is much smaller than expected within quark models; even worse, the
existence of many known states has been challenged in a recent analysis which
includes - compared to older analyses - high-precision data from meson
factories. Hence $\pi N$ elastic scattering data do not provide a well-founded
starting point of any phenomenological analysis of the baryon excitation
spectrum. Photoproduction experiments now start to fill in this hole. Often,
they confirm the old findings and even suggest a few new states. These results
encourage attempts to compare the pattern of observed baryon resonances with
predictions from quark models, from models generating baryons dynamically from
meson-nucleon scattering amplitudes, from models based on gravitational
theories, and with the conjecture that chiral symmetry may be restored at high
excitation energies. Best agreement is found with a simple mass formula derived
within AdS/QCD. Consequences for our understanding of QCD are discussed as well
as experiments which may help to decide on the validity of models.
