Optical coherence tomography (OCT) is a noninvasive imaging modality that uses low-coherent interferometry to visualize an optical cross-section of biological tissues. Over the past two decades, the ability to perform OCT imaging of the neural retina has afforded clinicians and researchers a highly reproducible method of diagnosing and following diabetic macular edema (DME) that compares favorably to other methods of DME assessment such as clinical examination and fundus photography. Although central subfield mean thickness has been the OCT parameter most commonly used to evaluate DME in clinical research trials, OCT also allows evaluation of morphologic changes that occur in DME, including compact retinal thickening, intraretinal cystic changes, subretinal fluid, and vitreomacular traction. OCT parameters have been shown to be only moderately correlated with visual acuity. However, improvements in technology leading to higher resolution, faster acquisition speed, image registration, and three-dimensional imaging that are available with newer spectral domain OCT models may allow future identification of valid OCT-derived surrogate markers for visual function in patients with diabetes.