Despite the extensive literature concerning atrial arrhythmias, there are relatively few articles on the anatomy of the atrial chambers. Since electrophysiological mapping and interventional treatments of atrial arrhythmias involve entering the chambers, this article reviews the gross structures to provide a better understanding of the atriums, the septum, and the connecting great veins. In addition, based on the human heart, differences between porcine and canine hearts are highlighted. The right and left atriums are characterized by morphologically distinct appendages. The right atrium contains prominent muscular bundles and an extensive array of pectinate muscles. The distal ramifications of the terminal crest lead to the "flutter" isthmus. By contrast, the left atrium has relatively smooth walls. The atrial septum is limited to the valve of the oval fossa and its immediate muscular rim. Atrial musculature extends beyond the veno-atrial junctions to the outside of the pulmonary veins. The longest sleeves are around the upper pulmonary veins, and similar sleeves are seen around the superior caval vein. The structure of the atrium is more than an anatomic curiosity. It has practical implicationsfor mapping and interventional procedures.