What do we learn from (e,e'p) experiments

The promise and limitations of studying nuclear structure and reactions using (e,e'p) reactions are discussed. A(e,e'p)A-1 reactions on complex nuclei are used to test new relativistic mean-field calculations and their ingredients such as ground-state wave function, current operators, final-state interactions and two-body currents, and to assess the limits of single-particle models. For higher excitations of the residual nuclei, A(e,e'p) reactions reveal only qualitative features of the reaction mechanism, and their increased usefulness awaits input from more complex reactions such as A(e,e'X). The (e,e'p) experiments on few-body nuclei provide essential input to quickly developing theories for few-body systems, where individual theoretical ingredients can be studied very selectively. A large data set is needed for this purpose, and existing or proposed experimental programs are discussed. If hadronic theories can meet the challenge of consistently describing this body of data at low and moderate energies, the transition from hadronic to quark-gluon degrees of freedom could be probed in experiments at higher energies.