The rhythm of the heart, its normal functioning and pathologic disturbances, has been a favored subject of investigation by clinical and basic scientists in recent decades. This heightened interest and attention was stimulated by the somber and surprising revelations from epidemiologists and pathologists of the enormity of the number of sudden arrhythmic deaths in the Western world, and the concurrent advancement of technology for recording and control of electrical activity of the heart. Technological advancements have included the recording of intracellular potentials from cardiac cells, the recording of intracardiac extra cellular potentials generated by specific cardiac structures, simultaneous record ings from numerous sites with computer processing for spatial mapping of activa tion or potential variations with time, high gain, high resolution recordings with signal averaging for detecting potentials of low amplitudes, complex stimulation protocols, various high energy stimulation modes, intracellular voltage control of multicellular preparations and single cardiac cells, and the isolation of single cardiac cells for electrophysiological study. The interest and technology have produced an increasing bounty of information and understanding, acceptable solutions to some clinical problems, and definite progress toward solutions to other problems. Progress in research in electrophysiology and arrhythmias has been reviewed and highlighted in various meetings and books in recent years. Because the body of information has become so large, general overviews of the field have neces sarily been superficial in certain aspects or have contained gaps.