Diabetes (lat. Diabetes mellitus) is a heterogeneous disorder that occurs as a result of interference of different pathological conditions,genetic susceptibility and environmental change greatly contribute to disease. Diabetes is characterized by high blood glucose levels, resulting in a condition in which the pancreas stops completely or partly produce the hormone insulin or the insulin is not effective in the body. There are two main types of diabetes, type 1 (insulin dependent) and type 2 diabetes (insulin resistance). With two main types there is a third, gestational diabetes, a special form that occurs during pregnancy. Disturbance of homeostasis in cells of the pancreas, liver and adpose tissue is the result of accumulation of unfolded and misfolded polypeptide chains in the lumen of the endoplasmic reticulum (ER). Consequence of this situation is ER stress that plays a significant role in the development of pathological conditions such as diabetes. The answer to stress ER is preserved cell response known as the "unfolded protein response" (UPR). UPR is primaril y directed toward establishing homeostasis, and in conditions of prolonged stress causes ER apoptosis leading to a large loss of β-cells of the pancreas. Besides leading to loss of β-cell ER stress also causes insulin resistance as a consequence of interference of signal molecules sensory pathways and ER stress signaling pathways of insulin. The consequence of insulin resistance is poor inhibition of gluconeogenesis and disturbed glucose uptake in muscle and poorly inhibited lipolysis in adipose tissue. Development of the disease includes a number of different mechanisms and ER stress is one of the main mechanisms that cause diabetes.