Atrial fibrillation (AF) is the most common cardiac arrhythmia, which is associated with substantial risk of stroke and thromboembolism. The epidemiology and health care burden associated with AF have increased significantly, and will continue to rise. Until recently, the concept and/or quantification of disease burden in AF tended to be ignored nor its consequences recognised. However, AF burden can now be assessed more accurately and reliably with the aid of cardiac rhythm management devices. There is a lot of interest on the issue of 'how much AF is needed to cause thromboembolism?' and this article summarises the available literature on this topic, with the aim of providing a better understanding of the clinical importance of device-detected atrial high-rate episodes and an overview of arrhythmia burden on thrombogenesis and clinical thromboembolism.