A cooling system for an internal combustion engine, the cooling system comprising: a radiator; a main cooling passage (114a, 114b) provided within an engine housing (2, 4) of the engine, the main cooling passage configured to carry a bulk flow of coolant to cool the engine housing; a main cooling duct configured to provide coolant from the radiator to the main cooling passage and to receive coolant from the main cooling passage; at least one additional cooling duct (126a, 126b), wherein the additional cooling duct branches from the main cooling duct downstream of the radiator; and one or more additional cooling passages (124a, 124b) provided within the engine housing, the or each additional cooling passage configured to introduce a flow of coolant midstream into the flow of coolant in the main cooling passage, wherein the additional cooling passages receive coolant from the radiator via the additional cooling ducts; wherein the engine housing comprises one or more high temperature regions (120a, 120b), which are at a higher temperature than one or more low temperature regions of the engine housing; and wherein the additional cooling passages are configured to direct the introduced coolant towards the one or more high temperature regions, wherein the cooling system further comprises one or more additional coolant pumps (130a, 130b) provided in each additional cooling duct, the additional coolant pumps configured to pump the coolant passing through the additional cooling passages, characterised in that the flow of coolant within the main cooling passage is driven by thermosyphoning, in such a way that no pump is provided in the main cooling duct and in the main cooling passage. a radiator; a main cooling passage (114a, 114b) provided within an engine housing (2, 4) of the engine, the main cooling passage configured to carry a bulk flow of coolant to cool the engine housing; a main cooling duct configured to provide coolant from the radiator to the main cooling passage and to receive coolant from the main cooling passage; at least one additional cooling duct (126a, 126b), wherein the additional cooling duct branches from the main cooling duct downstream of the radiator; and one or more additional cooling passages (124a, 124b) provided within the engine housing, the or each additional cooling passage configured to introduce a flow of coolant midstream into the flow of coolant in the main cooling passage, wherein the additional cooling passages receive coolant from the radiator via the additional cooling ducts; wherein the engine housing comprises one or more high temperature regions (120a, 120b), which are at a higher temperature than one or more low temperature regions of the engine housing; and wherein the additional cooling passages are configured to direct the introduced coolant towards the one or more high temperature regions, wherein the cooling system further comprises one or more additional coolant pumps (130a, 130b) provided in each additional cooling duct, the additional coolant pumps configured to pump the coolant passing through the additional cooling passages, characterised in that the flow of coolant within the main cooling passage is driven by thermosyphoning, in such a way that no pump is provided in the main cooling duct and in the main cooling passage.