I. Introduction
Over the last years, Grid computing evolved from vision to reality. Researchers use Grid infrastructures to access super-computing and cluster systems. To realize this unified access, a Grid resource manager has to be deployed to bridge the boundaries of institutions and divisions. While this development may lead to significant cost reduction it comes with some concerns. Most of these challenges and concerns can be dealt with or fend off by the introduction of Quality-of-Service (QoS) for the user.