Despite atrial fibrillation (AF) being the commonest cardiac arrhythmia, much less is known regarding atrial physiology in comparison to the well studied ventricle. The transverse (t)-tubule is a deep invagination of the surface membrane which forms a regular network in mammalian ventricular myocytes but is generally absent from the atria of small mammals e.g. rat. Proteins that couple excitation to the rise in intracellular Ca²⁺ are concentrated around the t-tubule membrane and thus intracellular Ca²⁺ rises rapidly and synchronously throughout the ventricular myocyte but much more slowly in the rat atrial myocyte. We have used confocal microscopy and electrophysiological experiments to investigate the role of atrial t-tubules in health and disease. Unlike the rat, large mammalian atria (including human) possess t-tubules resulting in a more rapid rise in intracellular Ca²⁺ in sheep atria. In a sheep model of heart failure (HF) we have shown atrial t-tubules are almost completely lost and this is associated with a slowing in the rise of intracellular Ca²⁺. Electrophysiological experiments have highlighted how t-tubule loss and slowed Ca²⁺ release affect atrial Ca²⁺ homeostasis in failing hearts. Firstly, peak L-type Ca²⁺ current and amplitude of the systolic Ca²⁺ transient were reduced. We were able to mimic this result by artificially removing t-tubules from control atrial myocytes. Secondly we observed an increase in SR Ca²⁺ content in atrial cells from the failing heart despite a reduction in SR mediated uptake. Both increased SR Ca²⁺ content, and decreased SR uptake have the potential to be arrhythmogenic and therefore may contribute to the propensity for AF in HF.