T2K is a long baseline neutrino experiment which uses a beam of muon neutrinos, produced at J-PARC and detected at Super-Kamiokande, to study the neutrino oscillation parameters. The measurement of cross-sections in the T2K energies can constrain the uncertainties on the model predictions and help the oscillation analyses reach the necessary sensitivity to measure CP violation in the lepton sector. This thesis describes the measurement of the CC1 + cross-section in water using Run II-IV T2K data. The T2K near detector, ND280, is used to select a sample of CC1 + events having vertices in the water layers of the downstream ne-grained detector (FGD). The Time Projection Chambers (TPC) are used for the particle identi cation and to measure their momenta. The Electromagnetic Calorimeters (ECals) are used to reject events that produce electromagnetic showers coming from neutral pions. A Bayesian unfolding method with background subtraction and two control samples is used to extract the cross-section. The control samples constrain the background coming from interactions on carbon and deep inelastic scattering. The single di erential cross-section is presented as a function of the muon kinematics, the pion kinematics, the angle between the muon and the pion, and the reconstructed neutrino energy. A future long baseline experiment between J-PARC and Hyper-Kamiokande is presented as a natural continuation to the T2K experiment. Hyper- Kamiokande will be a next generation water Cherenkov detector with a total ( ducial) mass of 0.99 (0.56) million metric tons. A total exposure of 7.5MW 107 sec integrated beam power will lead to the measurement of CP to better than 19 degrees for all possible values of CP . CP violation in the lepton sector could be established at better than 3 (5 ) for 76% (58%) of the CP parameter space.