The growth of the first dark matter halos into the first massive structures largely happens at z>2-4, when massive elliptical galaxies were forming, while baryons were filling these structures and strongly interacting with the forming galaxy populations. Understanding galaxy formation in the distant Universe is crucially connected to the study of the earliest ’forming’ clusters. High redshift galaxy clusters (z>1.5, thereafter high-z clusters) are very sensitive tracers of large-scale structure formation and evolution processes and can help understanding some of the most striking problems in today’s cosmology and extragalactic astrophysics.

1- From the point of view of cosmology and dark matter haloes, high-z clusters may yield information on non- Gaussianities of primordial fluctuations, and can challenge the Λ Cold Dark Matter (ΛCDM) model; In particular, they may help answering the tricky question of the formation of the first dark matter haloes. Finally, clusters at lower and high-redshift have baryonic processes at play not understood and limiting the accuracy in the use of clusters scaling relations for constraining cosmological parameters.

2- From the standpoint of galaxy formation and evolution, studying massive dark matter halos may provide a wealth of observational constraints on the kinematics and evolutionary history of galaxies in massive galaxy clusters. Key questions are: what is the fate of the baryons in these early structures ? What drives the star-formation efficiency and morphological transformations ?

3- From the astrophysical processes point of view, a key question deals with the interplay of the gas. It accounts for most baryons, but many processes like pre-heating and energy injection impact the Mass Function and have implications at larger scales. In particular: how feedback affects galaxy activity of star formation vs AGN ? What is the fate of the gas ?

The search for high-z clusters and their detailed physical study can address these questions, and is a new, rapidly evolving and highly competitive field. This workshop aims at 1) providing a state-of-the art statement of current research; 2) favoring discussions, exchanges and possibly new collaborations; 3) identify key directions for future studies.