Extreme Light Infrastructure - Nuclear Physics will host two major research systems with features that go well beyond the present-day state of the art: a 2×10 PW high-power lasers system and a high brilliance gamma beam system. The facility will open new opportunities for nuclear physics research in fields such as laser-driven nuclear physics, tools and method for the diagnostics of laser-matter interaction, nuclear photonics, nuclear astrophysics, photo-fission and production of exotic nuclei, applications in industry, medicine, space science. The present paper summarizes the main characteristics of the ELI-NP research systems and some of the key nuclear physics topics to be addressed with gamma beams.