  An experimental observation of proton decay would be a spectacular proof of
grand unification. Currently, the best constraint on the proton lifetime for
the p->e+ pi0 decay channel, coming from the Super-Kamiokande experiment,
reaches 8*10^33 years. To improve the measurement, much bigger detectors should
be constructed. Moreover, a better description of the bound-nucleon states and
of the propagation of the proton-decay products through nuclear matter have to
be developed. In this article special attention is paid to the argon nucleus
because a liquid argon detector is a promising candidate for the future large
apparatus.
