Polydipsia is a reasonably common clinical sign reported by owners of older cats. Most commonly polydipsia occurs as a compensatory response to polyuria, and therefore polyuria and polydipsia are usually grouped together when considering differential diagnoses and the diagnostic approach. Regardless of the cat’s lifestyle, polydipsia is more frequently the clinical sign noticed by owners and is therefore the focus in this chapter. The chapter will look at water intakeand approach to diagnosis.