A heat exchanger for a gas turbine engine comprising: a first plate for a coolant medium, the first plate including a first cool portion, a second cool portion, a coolant inlet and a coolant outlet, the coolant inlet and the coolant outlet disposed on a common side, and a second plate in thermal communication with the first plate, each of the plates (70, 72) includes a plurality of channels that define a flow path for the corresponding flows, spacer bars are provided within each of the plates (70, 72) to define a direction of flow through the channels, the spacer bars also define the number of passes that each flow will make for a defined area, the first cool portion includes more spacer bars than is provided in the second cool portion such that the first cool portion includes more passes of the coolant medium than the second cool portion, the heat exchanger being characterised in that: the second plate includes a first hot portion including a first inlet for a flow of air and a first outlet and a second hot portion including a second inlet for a flow of lubricant and a second outlet, wherein the first cool portion is in thermal communication with the first hot portion and the second cool portion is in thermal communication with the second hot portion; wherein the first hot flow comprises a flow of air and the second hot flow comprises a flow of oil; and the first cool portion and the first hot portion have a common width (92, 112) and a common length (90, 110) in parallel planes. a first plate for a coolant medium, the first plate including a first cool portion, a second cool portion, a coolant inlet and a coolant outlet, the coolant inlet and the coolant outlet disposed on a common side, and a second plate in thermal communication with the first plate, each of the plates (70, 72) includes a plurality of channels that define a flow path for the corresponding flows, spacer bars are provided within each of the plates (70, 72) to define a direction of flow through the channels, the spacer bars also define the number of passes that each flow will make for a defined area, the first cool portion includes more spacer bars than is provided in the second cool portion such that the first cool portion includes more passes of the coolant medium than the second cool portion, the heat exchanger being characterised in that: the second plate includes a first hot portion including a first inlet for a flow of air and a first outlet and a second hot portion including a second inlet for a flow of lubricant and a second outlet, wherein the first cool portion is in thermal communication with the first hot portion and the second cool portion is in thermal communication with the second hot portion; wherein the first hot flow comprises a flow of air and the second hot flow comprises a flow of oil; and the first cool portion and the first hot portion have a common width (92, 112) and a common length (90, 110) in parallel planes. the second plate includes a first hot portion including a first inlet for a flow of air and a first outlet and a second hot portion including a second inlet for a flow of lubricant and a second outlet, wherein the first cool portion is in thermal communication with the first hot portion and the second cool portion is in thermal communication with the second hot portion; wherein the first hot flow comprises a flow of air and the second hot flow comprises a flow of oil; and