  After discussion of mechanisms of lepton number violation, we consider meson
decays $K^+ \to\pi^ - \ell ^ + \ell '^ +$ and $D^ + \to K^ - \ell ^ + \ell '^
+$ ($\ell,\ell ' = e,\mu $) with $\Delta L= 2$ in the Standard Model extended
by massive Majorana neutrinos and in a supersymmetric extension of the Standard
Model with explicit breaking of R-parity by trilinear or bilinear Yukawa
couplings in the superpotential. We give estimates of the branching ratios for
these decays and compare the effectiveness of various decay mechanisms taking
into account present experimental bounds on lepton mixing, masses of neutrinos
and superparticles, and R-parity violating couplings.
