A thermal management system for a gas turbine engine, comprising: a first heat exchanger configured to provide a first conditioned fluid; a second heat exchanger in series flow communication with said first heat exchanger and configured to exchange heat with at least a portion of said first conditioned fluid to provide a second conditioned fluid; and a third heat exchanger in parallel flow communication with said second heat exchanger and configured to selectively exchange heat with a portion of said first conditioned fluid to provide a third conditioned fluid, characterised in that the thermal management system further comprises: a first passage configured to communicate a first portion of said first conditioned fluid to said second heat exchanger; a second passage configured to communicate a second portion of said first conditioned fluid to said third heat exchanger; a third passage configured to receive said second conditioned fluid and said third conditioned fluid to provide a mixed conditioned fluid; a fourth passage configured to receive a first portion of said mixed conditioned fluid having a first temperature for communication to a first engine system; and a bypass passage extending between said first heat exchanger and a fifth passage, the fifth passage configured to receive a second portion of said mixed conditioned fluid for communication to a second engine system at a second temperature, wherein the second portion of said mixed conditioned fluid is mixed with a third portion of the first conditioned fluid, the third portion being communicated along the bypass passage, wherein said second temperature is greater than said first temperature. a first heat exchanger configured to provide a first conditioned fluid; a second heat exchanger in series flow communication with said first heat exchanger and configured to exchange heat with at least a portion of said first conditioned fluid to provide a second conditioned fluid; and a third heat exchanger in parallel flow communication with said second heat exchanger and configured to selectively exchange heat with a portion of said first conditioned fluid to provide a third conditioned fluid, characterised in that the thermal management system further comprises: a first passage configured to communicate a first portion of said first conditioned fluid to said second heat exchanger; a second passage configured to communicate a second portion of said first conditioned fluid to said third heat exchanger; a third passage configured to receive said second conditioned fluid and said third conditioned fluid to provide a mixed conditioned fluid; a fourth passage configured to receive a first portion of said mixed conditioned fluid having a first temperature for communication to a first engine system; and a bypass passage extending between said first heat exchanger and a fifth passage, the fifth passage configured to receive a second portion of said mixed conditioned fluid for communication to a second engine system at a second temperature, wherein the second portion of said mixed conditioned fluid is mixed with a third portion of the first conditioned fluid, the third portion being communicated along the bypass passage, wherein said second temperature is greater than said first temperature. a first passage configured to communicate a first portion of said first conditioned fluid to said second heat exchanger; a second passage configured to communicate a second portion of said first conditioned fluid to said third heat exchanger; a third passage configured to receive said second conditioned fluid and said third conditioned fluid to provide a mixed conditioned fluid; a fourth passage configured to receive a first portion of said mixed conditioned fluid having a first temperature for communication to a first engine system; and a bypass passage extending between said first heat exchanger and a fifth passage, the fifth passage configured to receive a second portion of said mixed conditioned fluid for communication to a second engine system at a second temperature, wherein the second portion of said mixed conditioned fluid is mixed with a third portion of the first conditioned fluid, the third portion being communicated along the bypass passage, wherein said second temperature is greater than said first temperature.