  This paper was written as part of a book entitled: "Questions of Modern
Cosmology - Galileo's Legacy" which is a celebrative book dedicated to Galileo
Galilei. The book is published in 2009, the International Year of Astronomy,
since it is intended to be a modern tribute to the astronomer who, 400 years
ago, first pointed a telescope towards the night sky. The book is written in
the form of interviews between the editors and many physicists, astrophysicists
and cosmologists from all over the world. The editors engaged in several
discussions on the formation and evolution of the Universe with the aim of
summarizing the most important and significative advances made by cosmology
over the past century and at the beginning of the new millennium. This paper
deals with X-ray clusters of galaxies and how they can be used to constrain
fundamental cosmological parameters.
