I discuss current large surveys of galaxy clusters in the X-ray wavebands at medium depth, with special emphasis on SPIDERS (The Spectroscopic Identification of eROSITA Sources). It is one of the most ambitious observational programs aimed towards detecting, confirming, cataloguing and interpreting large samples -- several thousands -- of X-ray detected galaxy clusters, currently detected in ROSAT data. The goal is precision cosmology and galaxy cluster population studies. I introduce some interesting scientific challenges faced in this enterprise.