  The MSW (Mikheyev-Smirnov-Wolfenstein) effect is the effect of transformation
of one neutrino species (flavor) into another one in a medium with varying
density. Three basic elements of the effect include: the refraction of
neutrinos in matter, the resonance (level crossing) and the adiabaticity. The
key notion is {\it the neutrino eigenstates} in matter. Physical picture of the
effect is described in terms of the flavors and the relative phases of
eigenstates and the transitions between eigenstates. Features of the large
mixing realization of the MSW effect are discussed. The large mixing MSW effect
(LMA) provides the solution of the solar neutrino problem. We show in details
how this mechanism works. Physics beyond the LMA solution is discussed. The
lower $Ar$-production rate (in comparison with the LMA prediction) and absence
of significant "turn up" of the spectrum at low energies can be due to an
additional effect of the light sterile neutrino with very small mixing.
