Background: In past decades, our understanding of the vasopressin-mediated renal concentration mechanism has improved considerably due to the discovery of new endocrine-related neurotransmitters and the elucidation of the roles of crucial molecular players in this process. The identification of disease-causing genes in hereditary disorders of water balance has been extremely helpful in identifying these pivotal molecules. Acquired diseases, however, are the most frequent causes of impaired water homeostasis. Diagnosis: The clinical and biochemical diagnosis of hormonal deficit is confirmed by standard laboratory tests, but recent advances in imaging techniques have shed new light on the pathophysiology of many of these diseases. Management: Magnetic resonance imaging (MRI) permits identification of the posterior pituitary in vivo and can clearly delineate the shape, size and enhancement pattern of the pituitary stalk, as well as its functional integrity. Thickening of the pituitary stalk is a common finding on MRI scans in several pituitary stalk pathologies, but it is not specific to any single pathological subtype. However, biopsy of an enlarged pituitary stalk should be reserved for selected patients with hypothalamic-pituitary mass or with progressive thickening of the pituitary stalk.