Diabetes is a heterogeneous disease that leads to hyperglycemia. Long-term complications of the disease have serious consequences on the patient's health and on costs in the health care system. It is important to diagnose, treat and control the disease early. The diagnosis is made on the basis of glucose values in the fasting venous plasma, the concentration in the venous plasma 2 hours after the glucose load, the random glucose values in the plasma with the present symptoms and on the basis of the HbA1c value. We also distinguish between impaired fasting glucose and impaired glucose tolerance when glucose values do not yet indicate diabetes, but are too high to be considered normal. Glycemic control is performed by measuring HbA1c, self-monitoring of blood glucose and continuous glucose monitoring. HbA1c is the standard biomarker of glycemic control and indicates the average glycemia of the last 120 days. Self-monitoring of blood glucose is mostly used by patients who are on insulin therapy and is used several times a day. Continuous glucose monitoring enables monitoring of the concentration and trend of glucose in real time. A lot of information is obtained from continuous glucose monitoring, which is combined into an ambulatory glucose profile and enables a personalized diabetes treatment plan. In order to diagnose the disease earlier and to make the monitoring of the disease as successful as possible, other markers such as microRNA, tear film, nail proteins, and saliva are also being investigated.