Grids are becoming mission-critical components in research and industry, offering sophisticated solutions in leveraging largescale computing and storage resources. Grid resources are usually shared among multiple organizations in an opportunistic manner. However, an opportunistic or "best effort" quality-of-service scheme may be inadequate in situations where a large number of resources need to be allocated and applications which rely on static, stable execution environments. The goal of this work is to implement what we refer to as quasi-opportunistic supercomputing. A quasi-opportunistic supercomputer facilitates demanding parallel computing applications on the basis of massive, nondedicated resources in grid computing environments. Within the EU-support project QosCosGrid we are developing a quasi-opportunistic supercomputer. In this work we present the results obtained from studying and identifying the requirements a grid needs to met in order to facilitate quasi-opportu...