Cushing's disease is a very common endocrine disease in dogs. It is a multiple organ dysfunction caused by chronically elevated concentration of glucocorticoids, hormones that are secreted into the bloodstream by the adrenal cortex. The disease is also known as Cushing's syndrome or hypercortisolism. The adrenal gland functionally and embryologically comprises two distinct parts, a cortex and a medulla. The cortex itself, which is functionally and structurally divided from the medulla. is subdivided into three zones, all of which produce different types of hormones: the zona arcuata is responsible for the synthesis of mineralocorticoids, the zona fasciculata synthesizes and secrets glucocorticoids, while the zona reticularis produces hormones called androgens. Mineralocorticoids help regulate electrolytes and water balance in the body; glucocorticoids, which stimulate gluconeogenesis, are involved in the metabolism of proteins and fats, while at the same time having anti-inflammatory effects. The function of the adrenal cortex is regulated by CRH and ACTH, the former originating from hypothalamus and the latter from adenohypophysis. Hyperadrenocorticism in dogs may occur spontaneously or may be iatrogenic. Spontaneously occuring hyperadrenocorticism is classified as: pituitary-dependent hyperadrenocorticism and adrenal-dependent hyperadrenocorticism. Pituitary-dependent hyperadrenocorticism is the most common form of spontaneous hyperadrenocorticism in dogs and accounts for 80 – 85 % of cases in dogs. Adrenal-dependent hyperadrenocorticism affects the remaining 15 – 20 % of dogs. Iatrogenic hyperadrenocorticism is caused by excessive administration of glucocorticoids used in regulation of allergies and autoimmune diseases. Certain breeds of dogs are more prone to develop Cushing's syndrome and a slightly greater incidence has been found in spayed female dogs. Hyperadrenocorticism is most commonly diagnosed in older dogs, especially in small breed dogs. Clinical manifestations of this disease are numerous and veriable. The most common 45 symptoms include the combination of the following clinical signs: polydipsia and polyuria, polyphagia, abdominal distention, liver enlargement, weakness and muscle atrophy, letarghy and reduced physical activity, pathological skin changes, alopecia, failure to cycle in female dogs or testicular atrophy in male dogs, calcinosis cutis, myotonia, neurological signs and finally hypertensive retinopathy. There are different ways used to diagnose this disease, such as hematology tests, biochemical profile tests and urinanalysis, diagnostic imaging (X – rays, USG, CT, MRI), and endocrinology tests. Abdominal US and cranial CT/MRI scans have the most important role in its diagnosis, but the final diagnosis is made by the combination of screening endocrinological blood tests and diagnostic imaging. Pituitary-dependent hyperadrenocorticism is mostly treated medically with either trilostane or mitotane, but it can also be treated with radiation therapy or surgically. However, the latter two treatments are very rare. Adrenal-dependent hyperadrenocorticism is usually treated medically (by trilostane or mitotane), or surgically. The prognosis of patients with pituitary-dependent hyperadrenocorticism is very good if treated with correctly determined medication, and if a lesion does not expand to such an extent that it creates other complications. In that case, dogs can live for many years. The prognosis of patients with adrenal-dependent hyperadrenocorticism is excellent if tumor has been completely removed and has not metastasized. In case of inoperable tumors, if the disease is treated with mitotane, the prognosis is also good, and the prognosis is somewhat poorer if the disease is treated with trilostane. The prognosis of patients with Cushing's disease is favourable, and with an appropriate treatment dogs may live long and have a goodquality life.