Physics for a Polarized Electron-Nucleon/Nucleus-Collider

A number of physics arguments for a high-luminosity high energy polarized Electron- Nucleon/Nucleus-Collider (i.e. with a luminosity of at least 10^{33} cm^{-2} sec^{-1} and an invariant energy squarred of at least s >= 100 GeV^2) are presented. The main purpose of this machine would be generally speaking the investigation of QCD beyond the twist-2 level respectively of nuclear physics beyond the level of 'effective' models. Specific topics are: twist-2 and twist-3 spin-asymmetries as probes of both the internal hadron structure and the hadronization process, Off-Forward-Parton-Distributions as a new dimension of QCD-physics, nuclear effects for the nucleon structure and nuclear effects for QCD-dynamics in nuclei. The last two topics are also of direct relevance for high-energy heavy-ion-collisons. We conclude that the need for such a collider is clear, if nuclear physics is to continue its development towards a comprehensive understanding of QCD phenomena.