Atrial arrhythmias, including atrial fibrillation (AF), are characterised by irregular and rapid electrical activation of the atria. The mechanisms underlying the initiation and maintenance of AF, however, are incompletely understood. It is believed that the electrical heterogeneity and structural anisotropy of the atria play an important role in generating and sustaining AF. In the past few years, we have developed a 3D anatomical model of the human atria and torso to investigate the underlying mechanisms of AF (Kharche et al., 2008; Colman et al., 2011; Aslanidi et al., 2011). The model considered a new family of cellular models for describing the regional differences in the electrical properties of the atria that include the sinoatrial node (SAN), left and right atrial appendage, atrial septum, pectinate muscle, crista terminalis, Bachmann bundles, pulmonary vein, atrioventricular ring, coronary sinus and atrioventricular node. It also considered the 3D anatomical structures that were constructed from visible human project. A recently reconstructed anatomical structure for the human SAN geometry and fibre structure (Chandler et al., 2011) was incorporated into the 3D atrial model. The atria model was then integrated into a torso geometry mesh and the forward problem was solved in order to simulate realistic ECG P-waves (Aslanidi et al., 2011). The developed model was validated by quantitatively comparing the simulated atrial activation patterns to experimental data in both normal and abnormal conditions (Lemery et al., 2007; Kistler et al., 2006), and comparing the simulated body surface potential (BSP) to experimental BSP maps (Mirvis, 1980) and ECG P-waves under normal conditions. The model was then used to investigate the role of atrial electrical heterogeneity and anisotropy in initiation and maintenance of re-entrant excitation wave induced by the standard S1-S2 stimulus protocol. It was shown that while the electrical heterogeneity plays an important role for initiating re-entrant excitation waves, the structural anisotropy is the key for sustaining these re-entrant wavelets (Aslanidi et al., 2011). In conclusion, a validated multi-scale biophysically detailed model of the human atria and torso has been developed, providing a powerful platform for studying AF mechanisms.