Coronary collateral artery (CCA) is an alternative source of blood supply to an ischemic myocardial region. It is generally believed that there is a beneficial effect of collaterals in patients with coronary artery disease (CAD). Recently, reliable methods have become available for the assessment of the adequacy of collateral flow. However, the debate regarding the importance of CCA in the different clinical settings continues. In this review article, we discuss about the functional relevance of CCA to provide an overview of its impact on the prognosis of patients with CAD.