Introduction: The evolving cluster and protocluster population.

In this introductory talk, I place clusters and protoclusters, and their evolution, in the general context of the cosmic web formation. I also give an overview of the technics used to find clusters and their progenitors, and of the coverage of the redshift-mass plane achieved with current samples. I summarize the open questions that can be addressed from local merging cluster observations, statistical studies of the dynamical state of the cluster population, the cluster outskirts and protoclusters.