The goal of the SISCI-Pthreads project is the implementation of a completely transparent, POSIX compliant thread package on top of clusters of PCs. This will not only allow users to run multithreaded applications on loosely coupled systems in the same way as on SMPs, but will also introduce the shared memory programming model in an unrestricted fashion to clusters of PCs. Hence, porting efforts of existing multi-threaded applications will be reduced and this new architecture will be opened to a wide variety of new applications and users. In order to achieve this complete transparency in an efficient manner, this effort is based on a new interconnection technology, the Scalable Coherent Interface (SCI), which allows remote memory accesses in a cluster environment by providing a global physical address space.