Abstract: A physiological waveform detector such as for cardiac R-wave and blood pressure uses a comparator to compare the derivative of the waveform signal with a reference signal representing some percentage of the magnitude of the derivative of the waveform. If the derivative is of sufficient magnitude to trip the comparator a timing circuit begins counting. If the comparator remains tripped for a predetermined interval, the waveform is considered to be the desired one. Means are provided for detecting waveform peaks and valleys. The detector recognizes desired waveforms by two characteristics, one being magnitude and the other being duration.