The rare decays of a kaon into a pion and a lepton/antilepton pair proceed via a flavour changing neutral current and therefore may only be induced beyond tree level in the Standard Model. This natural suppression makes these decays sensitive to the effects of potential New Physics. I will motivate the use of lattice QCD in the calculation of hadronic contributions to rare kaon decay amplitudes, and discuss the techniques necessary to extract the physical amplitudes from lattice simulations.