I will discuss what we have learned from X-ray selected samples of distant X-ray luminous clusters of galaxies about cosmological parameters - Omega_matter in particular - and about how clusters form. The first results from a controlled experiment in simultaneous optical-X-ray searches for clusters of galaxies using completely automated methods will be described. Optical selection and X-ray selection techniques ``see'' different populations, bringing into question fundamental assumptions about clusters of galaxies as fair samples of the universe. The ROX cluster sample can reveal why this is so.
Received November 20, 2000