I discuss some theoretical aspects of how to observe leptonic CP violation. It is divided into two parts, one for CP violation due to Majorana, and the other more conventional leptonic Kobayashi--Maskawa (KM) phases. In the first part, I estimate the effect of Majorana phase to observable of neutrino-less double beta decay experiments by paying a careful attention to the definition of the atmospheric scale \Delta m². In the second part, I discuss Tokai-to-Kamioka-Korea two detector complex which receives neutrino superbeam from J-PARC as a concrete setting for discovering CP violation due to the KM phase, as well as resolving mass hierarchy and the \theta ₂₃ octant degeneracy. A cautionary remark is also given on comparison between various projects aiming at exploring CP violation and the mass hierarchy.

PACS numbers: 14.60.Pq, 14.60.Lm, 23.40.--s