  This lecture is an introduction to cosmological tests with clusters of
galaxies. Here I do not intend to provide a complete review of the subject, but
rather to describe the basic procedures to set up the fitting machinery to
constrain cosmological parameters from clusters, and to show how to handle data
with a critical insight. I will focus mainly on the properties of X-ray
clusters of galaxies, showing their success as cosmological tools, to end up
discussing the complex thermodynamics of the diffuse intracluster medium and
its impact on the cosmological tests.
