The propagation of electrical activity in cardiac tissue can be modelled by reaction diffusion equations, where a tensor of diffusion coefficients represents anisotropy due to fiber orintation, and excitation is represented by high-order, stiff differential systems. The effects of external electrical stimulation, as in artifical pacemakers, or in defibrillators, requires bidomain models, in wich intra- and extracellular currents are treated separately.simplified approaches are taken to this problem to illustrate two methods of defibrillation: by a sinhle large pulse, that eliminates all propagating activity, and by a series of smaller amplitude perturbations, that drive out re-entrant sources of excitation.