Clusters of galaxies are the largest bound gravitational systems in the universe. They outline the large-scale structure and thus are able to test predictions of cosmological models. Detailed studies of galaxy populations in clusters at high $z$ offer insights into galaxy evolution as a function of look-back time. Unfortunately, large homogeneous samples of galaxy clusters only exist at redshifts $<$0.5. There are around 1000 candidate clusters with photometric or otherwise estimated redshifts above 0.5, but spectroscopically confirmed clusters are still scarce in this redshift regime. Some clusters with $z>1$ have recently been identified from X-ray surveys.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html