Non-invasive biomagnetic measurements are feasible for obtaining functional information concerning the electrical activity of the human heart and brain. These methods have turned out to be promising in localizing various bioelectric sources in the body. For example, in magnetocardiographic studies of localizing arrhythmogenic tissue and both normal and abnormal conduction pathways between the atrial and the ventricles, the best accuracies reported are comparable to the results obtained by the invasive methods. We consider here basic principles of biomagnetic source localization methods, focusing on the magnetocardiographic mapping.