Hyper-Kamiokande, the next generation of water Cherenkov neutrino observatory, will open fascinating new window on the universe, including the search for Grand Unified Theory through proton decay, neutrino astronomy, and determination of the CP violation in the lepton sector. In the first part of this seminar, we will present some questions remaining in these various physics domains, as well as the answers we may be able to provide with Hyper-Kamiokande.