Atrial fibrillation (AF), a very common cardiac arrhythmia, has recently emerged as a new interdisciplinary epicenter of clinical, engineering, and basic science research. This emergence is fueled by new results from experiments, animal models of the disease, and multimodal measurements in patients, catalyzed by rapid developments in interventional approaches to treatment. Our understanding of the disease is growing but still inadequate to provide satisfactory patient care and so there is enormous opportunity for a range of research approaches. Some attributes of the disease also make it a simultaneously compelling, accessible, and challenging target for inquiry; AF emerges only as a result of long term changes in tissue substrate, is triggered by remote ectopic activity, involves one or more mechanisms of cardiac arrhythmias, has a variable time course in individuals, and is both damaging to immediate quality of life and a major risk for secondary disease.