Rare leptonic and semi-leptonic decays at LHCb

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 $J\mathrm{/}\psi \to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decay and the second is a comprehensive measurement of local and nonlocal contributions to the $B^0\to K^{\ast 0}{\mu }^{+}{\mu }^{-}$ decay.