A method and apparatus for monitoring recurrent waveforms such as electrocardiogram (e.c.g.) waveforms. The e.c.g. apparatus comprises integrators and stores for storing integrated segments of a normal, reference waveform complex. A timing circuit derives timing signals from the e.c.g. waveform to be monitored. The timing signals represent the time location of particular selected parts of the waveform which are to be compared with the corresponding parts of the stored waveform. The e.c.g. waveform is corrected by subtracting a version of the waveform after it has passed through a low pass filter from a delayed version. Corrected segments of the waveform are integrated and subtracted from the corresponding stored segments of the reference waveform to provide a difference signal and the difference signals are quantitatively summed. An output signal is produced if the total sum of the magnitudes of the difference signals exceeds a predetermined value to indicate an abnormal complex.