In this article we present the choices that the designers of any QRS detector must make and explain the constraints we adopted. We outline the signal processing that precedes and the beat analysis that follows QRS detection in our single-channel, arrhythmia-monitoring algorithm and then expound the QRS detection algorithm in detail. Finally, we present the results of a QRS detector performance evaluation and comment on their importance. This article can be read to three depths: the text affords an overview of QRS detection for on-patient, ambulatory arrhythmia analysis; the commented pseudocode documents the logic of our QRS detector; and the pseudocode "footnotes" supply technical detail.