Thoracic aortic dissection (TAD) is estimated to occur at a rate of 3–4 cases per 100,000 persons per year and is associated with a high mortality. Reported rates are probably underestimates of the true incidence of TAD because of difficulties in diagnosis. The incidence of TAD appears to have been increasing over time. TAD is most common in men and older individuals. Aortic dilatation is a well-established risk factor for TAD but is not a prerequisite; most ascending aortic dissections occur when aortic diameter is <5.5 cm. Although atherosclerosis and typical cardiovascular risk factors, such as hypertension and smoking, are associated with TAD, evidence supporting their direct causal role is lacking. Notably, diabetes mellitus is remarkably uncommon in patients with TAD. Other risk factors for TAD include inflammatory diseases, iatrogenic aortic injury, and drug use. Congenital cardiovascular defects, such as bicuspid aortic valve, and certain genetic syndromes, such as Marfan syndrome, are the genetic factors most commonly associated with TAD. Specific nonsyndromic genetic mutations in families and single nucleotide polymorphisms have also been identified as possible risk factors for TAD.