ROP is caused by abnormal vascular proliferation in response to hypoxia. The major risk factor for acute ROP is prematurity, as measured by birth weight and gestational age. In the developed world, ROP is usually observed in premature infants with a birth weight less than 1000 grams and/or a postmenstrual age of 31 weeks or less. Supplemental oxygen exposure also increases ROP incidence. Acute ROP is described according to the international classification of ROP. The classification scheme includes the location of the disease, the severity of the disease, the extent of disease, and the presence or absence of plus disease. ROP examinations require skill with indirect ophthalmoscopy and 360 degrees of scleral depression to fully examine the retina. Acute ROP is rarely confused with other retinal vascular diseases, particularly in the premature infant. The outcome of premature infants with ROP depends on the severity of disease. Mild disease tends to regress spontaneously with little sequelae. Rarely, ROP can progress to advanced stages and cause retinal detachment, retinal folds, macular ectopia, and severe vision loss. In cases of severe retinopathy, significant structural and functional impairment can occur even if therapy is initiated. Laser phototherapy or cryotherapy should be administered. The standard of care for the management of severe ROP is retinal ablation.