Dwarf galaxies represent the dominant population at high redshift and they most likely contributed in great part to star formation history of the Universe and cosmic reionization. The importance of dwarf galaxies at high redshift has been mostly recognized in the last decade due to large progress in observing facilities allowing deep galaxy surveys to identify low-mass galaxies. This population appear to have extreme emission lines and ionizing properties that challenge stellar population models. Star formation follows a stochastic process in these galaxies , which has important implication on the ionizing photon production and its escape fraction whose measurements are challenging for both simulations and observations. Outstanding questions include: what are the physical properties at the origin of such extreme properties ? What are the smallest dark matter halos that host star formation ? Are dwarf galaxies responsible for cosmic reionization ?