Optical coherence tomography (OCT) (Stratus OCT Model 3000, Zeiss-Humphrey Inc., Dublin, CA) is a computer-assisted precision optical instrument that generates cross-sectional images (tomograms) of the retina with ≤10 μm axial resolution (Fig. 12.1).1-8 Diabetic retinopathy is a major cause of visual impairment in the United States and around the world. 9-12 Diabetic macular edema (DME) is one of the main reasons for reduced visual acuity (VA) in patients with diabetic retinopathy. Macular edema affects approximately 29% of diabetic patients with a disease duration of 20 years or more, and is responsible for a significant degree of visual loss in this population.9 It involves a wide spectrum of pathologic changes, including diffuse or focal edema and cystoid macular edema, which may have hard exudate. Direct clinicopathologic correlation of these pathologic changes is unclear. Macular edema may be the first symptom of diabetic retinopathy and may be associated with proliferative or nonproliferative retinopathy (Fig. 12.2). More than half of these individuals will lose two or more lines of visual acuity within 2 years of follow-up.13 © 2009 Springer New York.