The interplay between the careful analysis of clinical electrocardiograms and results from animal experiments have in the past 60 years resulted in provo- cative and brill ant concepts on the mechanisms of cardiac arrhythmias in man. Many of the animal experiments however were done on open-chested dogs with cut cardiac nerves and under the influence of pharmaca. It is doubtful, therefore whether these results can be transferred without reser- vation to the human situation. The introduction of electrical stimulation of the heart in clinical cardiology has opened new ways to study some aspects of cardiac arrhythmias directly in the unaesthesized patient. This study reports observations on patients who were admitted to the University Department of Cardiology, Wilhelmina Gasthuis, Amsterdam, for the evaluation and treatment of tachycardias. Electrically induced premature beats were used in an effort to elucidate the origin and mechanism of these tachycardias. The first chapter is on classification and diagnosis of tachy- cardias with special emphasis on our current knowledge of the differential diagnosis between supraventricular tachycardias with aberrant conduction and ventricular tachycardias. This is followed by theoretical considerations on tachycardias especially in relation to the methods used in this study. After an outline of these methods the results of our studies in patients with atrial flutter, A-V junc- tional tachycardias and tachycardias related to the pre-excitation syndrome are reported. A discussion on the value of electrical stimulation for the treatment of tachycardias is followed by a summary of our results.