  In the last eight years, the Chandra and XMM-Newton satellites changed
significantly our view of X-ray clusters of galaxies. In particular, several
complex phenomena have been directly observed: interactions between cluster
galaxies and the Intra Cluster Medium (ICM), cold fronts in the ICM, hot
bubbles due to relativistic jets from radio loud AGN, the lack of cold gas in
``cool-cores'', and non-thermal X-ray emission. Still, this increasing
complexity does not prevent us from using X-ray clusters as a tool to constrain
cosmological parameters. In addition, observations of clusters up to redshift
~1.3 allowed us to trace the thermodynamical and chemical evolution of the ICM
on a time interval as large as 8 Gyr. In this presentation, I will give a
personal introduction to the most debated topics in this field, to end with
some prospects for the next-generation X-ray satellites.
