The combined results of the Planck satellite and the observations of the Gunn-Peterson absorption of distant quasars now point to a late reionization of the Universe around a redshift of z 8 to 6. Yet, the precise nature of the sources that powered one of the most important transitions in the history of the Universe remains elusive. Owing to an important observational progress in the recent years, early star-forming galaxies are now considered as the best candidates. However, at z>6, the census of relatively massive galaxies within reach of current facilities falls short of providing the required ionizing power to complete reionization. The only way to move forward is to use the gravitational lensing offered by massive galaxy clusters to detect the faintest and more abundant population of galaxies that could account for the remaining contribution of ionizing radiation. In this talk, I will review the studies enabled by “gravitational telescopes” and the latest results from the Hubble Frontier Fields program that aims to peer into the epoch of reionization to unprecedented depths. In light of these constraints, I will also discuss the need for a potential contribution from active galactic nuclei. Finally, I will briefly highlight few of the discoveries awaiting the combination of such lenses with the upcoming JWST.