In addition to ophthalmologic examination and retinal fundus photography, diagnostic modalities including fluorescein angiography (FA) and optical coherence tomography (OCT) currently play important roles in the evaluation of patients with diabetic retin- opathy (DR). Fluorescein angiography has been a major diagnostic procedure in the clinical evaluation of DR for several decades. This chapter will review the principles and methodology underlying fluorescein angiography, as well as its specific application to the evaluation of diabetic retinopathy and especially retinal vascular permeability. Although fluorescein angiography provides valuable anatomic and functional information pertaining to the retinal vasculature, it does not provide ultrastructural anatomic detail and does not allow quantification of retinal thickness. Optical coherence tomog raphy has emerged in recent years as an additional modality that provides clinically valuable information. This chapter details the principles and methodology of OCT. In addition, the application of OCT to the assessment of diabetic macular edema is discussed, including the value of OCT in providing quantitative information regarding retinal thickness. The different morphologic patterns of diabetic macular edema seen in OCT are presented, as well as a discussion of the application of OCT to clinical management of diabetic retinopathy. Finally, additional diagnostic modalities of potential benefit in evaluating DR are presented, including the retinal thickness analyzer (RTA) and scanning laser ophthalmoscopy (SLO). The detection of retinal abnormalities in diabetic patients is vital for preventing the associated complications and subsequent loss of vision. FA and OCT provide information essential to the optimal treatment and management of diabetic retinopathy.