Despite much recent theoretical and observational progress in our understanding of the formation of early cosmic structure, many fundamental questions remain open. The Epoch of Reionization (EoR) is an extremely fascinating period, as it hosts a number of important processes which decisively shape the structure of the Universe we see today.

Only recently have observations of very distant objects (quasars, galaxies, gamma ray bursts) and of the cosmic microwave background (CMB) radiation allowed quantitative studies of this important period in the evolution of our Universe. These suggest that, while the tail end of cosmic reionization can be positioned at z~5-6, the process must have begun at much earlier times and so must have had a complex history.

Although since the late 1950s it has been recognized that neutral hydrogen in the intergalactic medium may be directly detectable in emission or absorption against the CMB radiation at the frequency corresponding to the redshifted neutral hydrogen 21cm line providing a direct probe of the era of cosmological reionization, only now radio interferometers such as LOFAR, MWA and PAPER are delivering the first data.

At the moment though there is still a dearth of direct observational evidence and lots of questions remain unanswered: what was the nature of the first ionizing sources? Were the first stars much more massive than those we observe today, and when/why did the transition from the so-called top-heavy mode to a normal mode of star formation occur? What is the impact of these processes on cosmic reionization and the subsequent structure formation process? Were there other contributions to the ionizing photon production (e.g. mini-quasars or exotic particles)? These and many other questions regarding the end of the Dark Ages remain, at best, only partially answered and are still the subject of a lively debate. To better understand such fundamental processes much theoretical and observational investigation is still needed.

Within the coming year or two, much of the observational data that will be needed to answer some of the above still open questions will be available. The Planck CMB project would have delivered its data and products, including new constraints on the Thomson scattering optical depth of the CMB off free electrons released during the EoR. The long awaited 21cm data is also expected to give its first results, and provide a glimpse on the history of the reionization process. Projects designed to detect Lyman-alpha emitters from high redshift galaxies, such as the wide field survey carried out by the HyperSupremeCam on Subaru, will also be well on their way. Several projects planning to detect the Cosmic Infrared Background, such as Siber, will have given constraints on the contribution of the stars prevalent during the EoR.