The aim of this review is to summarize the available findings from previous research that has focused on retinal vascular caliber characteristics in diabetes and diabetic retinopathy and identify any gaps that exist in the current literature. A systematic Medline, EMBASE, and PubMed search of relevant articles was conducted with coverage up to the 30th of September, 2012. The search was not restricted by language but was limited to studies conducted in humans. The majority of articles conducted on children and adolescents with type 1 diabetes have reported that arterioles with larger caliber were present in the early stages of diabetic retinopathy (n = 5). Only a few studies conducted on older individuals with type 1 diabetes (n = 2) suggest that smaller retinal arteriolar caliber is associated with increased severity of diabetic retinopathy. Much stronger trends have been identified between venular caliber and older individuals with diabetes, with the vast majority of studies reporting that retinal venular dilation represents a later sign of severe diabetic retinopathy (n = 6), with only 1 study highlighting associations with incident diabetes (n = 1). Significant developments have occurred to better understand the relationship between retinal vascular caliber and the onset and progression of diabetes and diabetic retinopathy. Recent evidence suggests that retinal arteriolar dilation may be a possible risk factor in the early development diabetic retinopathy and retinal venules are dilated in persons with severe diabetic retinopathy. Despite this, the clinical significance of these findings requires further evaluation.