Unipolar stimulation-the passing of current through a single, small, extracellular electrode-is perhaps the simplest way to excite the heart. If we want to determine in general how electric currents affect cardiac tissue, we must begin by understanding unipolar stimulation. Our research over the past 10 years demonstrates that the anisotropic properties of cardiac tissue have profound implications during unipolar stimulation. Theoretical simulations using the bidomain model have predicted new and unexpected behavior, and have resolved several long-standing questions in cardiology. Experiments using electric, magnetic, and optical techniques have confirmed many of these theoretical predictions. In this article, we review this research and show how our combination of numerical simulations and experiments has fundamentally changed our understanding of unipolar stimulation of the heart © 1998 by Churchhill Livingstone.