A downhole tool is described comprising a housing (24), a fluid flow passage (28) located within (5) the housing (24), and at least one thermoelectric generator (30) located between the fluid flow passage (28) and the housing (24) and operable to generate an electrical output in the event of there being a temperature difference between, in use, the fluid within the fluid flow passage (28), and fluid in contact with the exterior of the housing (24).