Models of cardiac electrical activity cover a wide range of spatial scales, from the genesis of the ionic currents in individual cardiomyocytes to the generation of electrocardiograms on the torso. The level of detail that is appropriate and practicable depends on the problem investigated and the scope of the computations that are required. We briefly present three examples of modelling: the dynamics of the entrainment of a single cell, the impact of fibrosis on electrical propagation in a piece of tissue and the generation of ECG in human. In each case, the methods, results and limitations are discussed.