  Rare leptonic and semi-leptonic decays offer an excellent opportunity to test
the Standard Model of Particle Physics. These proceedings report on two recent
LHCb measurements. The first is an observation of the rare $J/\psi \to
\mu^{+}\mu^{-}\mu^{+}\mu^{-}$ decay and the second is a comprehensive
measurement of local and nonlocal contributions to the $B^0 \to
K^{*0}\mu^{+}\mu^{-}$ decay.
