We study supersymmetric extensions of the Standard Model with nonzero Majorana neutrino masses.
We concentrate on the snuetrino-antisneutrino oscillation and lepton number violation
phenomena. We found relations between model parameters and CPviolation observables. We show that in models with R-parity violation there are processes where there is possibility to observe CP violation in decays that are common to sneutrino and
anti sneutrino. We have studied the MSSM extended by Majorana sneutrino masses. In particular, we looked for ways to observe lepton number violation and CP violation phenomena via sneutrino anti-sneutrino oscillations. Wereviewed R-parity conserving models with CP symmetry. Then we turned to study an extension of this model where we impose neither R-parity nor CP symmetry on our model. Thus, we can accommodate imaginary parameters of CP violation in our formalism. First, we found the general formula for the number of parameters in MSSM lepton sector.We then calculated the splitting between the two light sneutrino masseigenstates for a model with broken CP symmetry. Chapter four wasdedicated to CP violation: formalism, possible sources of CP violationin our model and the calculation of CP violation parameters in termsof supersymmetry parameters. Next, we studied possible decay of thesneutrino and the anti-sneutrino via mixing with the Higgsbosons. Such decays requires R-parity violation . We obtained thedecay width difference between the two mass eigenstates of thesneutrino. We also proposed a way to measure the imaginary phases ofthe superpotential, by looking at the decay asymmetry in the twoflavor states of the sneutrino. We expressed these quantities in termsof the parameters of the model.