Conducting system potentials were first recovered from the body surface in dogs in 1973 using a sequence of high-gain amplification and filtering followed by digital signal averaging. Subsequent application of similar methods to man by more than 18 research groups world-wide have shown it feasible to record His Purkinje signals non-invasively in about 75% of subjects studied. Surface signals are of the order of 0.5-2.0 µV in amplitude. In 70-100% of cases they correlate well in time with the simultaneous internal His bundle electrogram but the pattern of surface signals varies from patient to patient. Their appearance fails to correlate with any specific underlying pathological change in AV conduction and is affected both by lead position and as a result of filtering. “High resolution” recordings made without signal averaging have been achieved in a few centres but with greater technical difficulty and a lower rate of signal recovery.