The risk for cardiovascular diseases and their complications, such as heart attack, stroke, chronic heart failure, and atrial fibrillation, is typically about twice as high in patients with diabetes mellitus. Furthermore, these complications not only occur more frequently but also earlier, and their prognosis is worse than in people without diabetes. Despite the relevance to prognosis, the number of unreported cases of diabetes in patients with cardiovascular diseases is very high, and therefore, they should be screened for diabetes. The intensity and strategy of diabetes therapy and other risk factors such as elevated LDL cholesterol, arterial hypertension, platelet inhibition, and anticoagulation depend on the respective risk constellation. In diabetes therapy, GLP-1 receptor agonists and SGLT-2 inhibitors are a preferred therapy strategy due to their HbA1c-independent cardioprotective effects. The consequences of ischemic cardiovascular diseases including chronic heart failure continue to be one of the most common causes of death and reasons for hospitalizations. Diabetes mellitus, with 8 million affected, is one of the most common chronic diseases in Germany with a simultaneously high number of unreported cases. Despite optimization of cardiovascular therapy strategies, diabetes continues to be associated with a typically twice as high risk. The reasons for this are still unclear and most likely associated with complex changes in the context of a metabolic syndrome or insulin resistance. Therefore, in this chapter, we first address the risks in diabetes, then diagnostics and therapy. In therapy, we focus on the drug therapy of diabetes in patients with cardiovascular disease or high risk. For the treatment of other risk factors such as elevated low-density lipoprotein (LDL) cholesterol or dyslipidemia, arterial hypertension, platelet inhibition, and anticoagulation, please refer to other chapters of this handbook, or see also Marx et al. (Braunwald’s heart disease: a textbook of cardiovascular medicine. Elsevier, Philadelphia, 2021).