Coronary artery aneurysms or ectasias are dilatations of arterial segments, which are typically incidentally identified at angiography. Several large clinical series have described the epidemiology and natural history of these malformations. The most common etiologic factor for coronary aneurysms is atherosclerotic coronary artery disease, but they can also be congenital or secondary to inflammatory or connective tissue disorders, with a well-known association with Kawasaki disease. Treatment may consist of surgical, percutaneous, or medical interventions. This review summarizes the current state of knowledge on coronary artery aneurysms and highlights important unresolved questions.