Microscopic optical potentials derived from free NN potentials

First principle optical potentials are studied with the nuclear matter approach, which is based on Brueckner's development of the many-body problem and the hole-line expansion ofHuefer and Mahaux. This contribution concentrates on the most recent developments in calculations of the Bethe-Goldstone reaction matrix with free NN potentials as input, referring to results from various authors together with our own results which are based on the Paris potential. As an important intermediate result we give a comprehensive discussion of the effective energy and density dependent interaction with an outline of the limitations so far detected. The non-relativistic nucle-on-nucleus optical potential for energies below 400 MeV is formulated as a lowest order folding model result including proper treatment of knock-on exchange effects. A status report on the microscopic optical potential is given via its application to various case studies. (author)