  In this talk I have raised the question of how the future discovery of
leptonic CP violation can be made robust even at accepting the rather large
current experimental uncertainties in our knowledges of neutrino propagation in
matter. To make progress toward answering the difficult question, I listed ways
to proceed: (1) Obtain tighter constraints on the MSW theory by testing it by
various neutrino experiments. (2) Measure the matter effect in situ, namely
within the experiment for discovering CP violation itself. (3) Uncover leptonic
CP violation in a matter effect free environment. I also reported a step made
toward the above point (2) by taking neutrino factory as a concrete setting; An
accurate in situ measurement of the matter effect looks promising.
