  Form factors, polarizabilities and excitation spectra are different aspects
of many-body structures. These aspects are constrained by sum rules based on
general principles like relativity, gauge invariance, causality and unitarity.
Several such structure effects of baryons and mesons will be reviewed, with
particular emphasis on the neutron charge form factor, the strangeness content
of the nucleon, polarizabilities, and the Gerasimov-Drell-Hearn and related sum
rules.
