Witnessing galaxy clusters: from maturity to childhood
  Galaxy clusters are the largest structures in Universe. They are very
important as both cosmological probes and astrophysical laboratories. Several
methods have been developed to detect galaxy clusters with different techniques
(optical, X-rays, Weak Lensing and Sunyaev-Zeldovich effect) providing cluster
samples with a well-characterized purity and completeness rates up to moderate
redshift (z$<$1.2). These samples allow us to study the systematic of different
methods and to obtain reliable mass estimations. On the contrary, high redshift
clusters only started to be explored very recently with the advent of deep IR
and X-ray data surveys, providing the first proto-clusters (z$>$1.5-2) ever
detected. In this talk, I introduce these techniques and review some of the
cluster samples obtained including particular striking cases. I discuss their
relevance in terms of cosmological and galaxy evolution constraints and
finally, I briefly refer to the cluster science predictions for the next
generation surveys.
