  The seesaw-extended MSSM provides a framework in which the observed light
neutrino masses and mixing angles can be generated in the context of a natural
theory for the TeV-scale. Sneutrino-mixing phenomena provide valuable tools for
connecting the physics of neutrinos and supersymmetry. We examine the
theoretical structure of the seesaw-extended MSSM, retaining the full
complexity of three generations of neutrinos and sneutrinos. In this general
framework, new flavor-changing and CP-violating sneutrino processes are
allowed, and are parameterized in terms of two $3\times 3$ matrices that
respectively preserve and violate lepton number. The elements of these matrices
can be bounded by analyzing the rate for rare flavor-changing decays of charged
leptons and the one-loop contribution to neutrino masses. In the former case,
new contributions arise in the seesaw extended model which are not present in
the ordinary MSSM. In the latter case, sneutrino--antisneutrino mixing
generates the leading correction at one-loop to neutrino masses, and could
provide the origin of the observed texture of the light neutrino mass matrix.
Finally, we derive general formulae for sneutrino--antisneutrino oscillations
and sneutrino flavor-oscillations. Unfortunately, neither oscillation phenomena
is likely to be observable at future colliders.
