A heater-cooler apparatus of an extracorporeal perfusion system comprises at least one fluid circuit (102, 104) providing a supply of a heat transfer fluid to the perfusion system, a cold storage unit (266), and a refrigeration unit (250) for charging the cold storage unit (266). The cold storage unit (266) comprises a chamber (312) containing a liquid that freezes at a temperature above that to which the heat transfer fluid is cooled by the refrigeration unit (250), and a passage through which the heat transfer fluid is conveyed, the passage extending through the chamber (312). This allows a more effective cold storage unit to be provided.