The syndrome of inappropriate antidiuresis (SIAD) is a condition characterized by increased secretion and/or effects of arginine vasopressin in the absence of appropriate triggers. This chapter discusses the prevalence, pathophysiology and aetiology of SIAD in dogs and cats, with reference to the condition in humans. It explores the clinical features, diagnosis, and treatment options for SIAD, highlighting the importance of water restriction and the potential use of vaptans and other medications. The document emphasizes the need for careful management of hyponatremia to avoid complications such as osmotic demyelination syndrome.