Adults with diabetes mellitus are much more likely to have cardiovascular disease than those without diabetes mellitus. Genetically engineered mouse models have started to provide important insight into the mechanisms whereby diabetes mellitus promotes atherosclerosis. Such models have demonstrated that diabetes mellitus promotes formation of atherosclerotic lesions, progression of lesions into advanced hemorrhaged lesions, and that it prevents lesion regression. The proatherosclerotic effects of diabetes mellitus are driven in part by the altered function of myeloid cells. The protein S100A9 and the receptor for advanced glycation end-products are important modulators of the effect of diabetes mellitus on myelopoiesis, which might promote monocyte accumulation in lesions. Furthermore, myeloid cell expression of the enzyme acyl-CoA synthetase 1 (ACSL1), which converts long-chain fatty acids into their acyl-CoA derivatives, has emerged as causal to diabetes mellitus-induced lesion initiation. The protective effects of myeloid ACSL1-deficiency in diabetic mice, but not in nondiabetic mice, indicate that myeloid cells are activated by diabetes mellitus through mechanisms that play minor roles in the absence of diabetes mellitus. The roles of reactive oxygen species and insulin resistance in diabetes mellitus-accelerated atherosclerosis are also discussed, primarily in relation to endothelial cells. Translational studies addressing whether the mechanisms identified in mouse models are equally important in humans with diabetes mellitus will be paramount.