The first part of this paper is a description of the design of low-pass, bandpass, high-pass, and notched filter difference equations using Z-transform techniques. The difference equation coefficient word lengths are significantly reduced by using a set of second-order difference equations. These equations have a form that minimizes computation. The second part of the paper contains applications of these filters to the processing of electrocardiograms (ECG). The filters are included as subroutines in a program that also includes an adaptive muscle-tremor filter, an “optimum” ECG estimator, and optimum estimators for the arrival time of various parts of the ECG waveform. Clinical results are presented. Copyright © 1968 by The Institute of Electrical and Electronics Engineers, Inc.