In preparing this chapter we have attempted to summarize structural concepts which underlie cardiac function. Considering those topics which may have the greatest relevance to cardiac toxicology, we selected four broad areas for discussion. We begin with cardiac embryology because it not only provides a foundation for basic macroscopic and microscopic structures, but also is a requisite for understanding congenital malfonnations. Next, the cardiocyte is dissected to reveal the structural correlates of its functions. Our intent here is not to cover the topic of cardiocyte organelles in any great detail, but rather to summarize the compartmentalization of this cell while stressing less commonly understood structure-function relationships. The third area, too often ignored, is the extracellular matrix, a component of the myocardium which is closely linked with both the cardiocyte and the coronary vasculature. Finally, the coronary vasculature is reviewed, with an emphasis on the microcirculation and its relationship to the cardiocyte.