A gas turbine engine, comprising: a buffer system (60; 160; 260) that communicates a buffer supply air (62; 162; 262A, 262B) to a portion of the gas turbine engine, wherein said buffer system (60; 160; 260) includes: a first bleed air supply (64; 164; 264A, 264B) having a first pressure; a second bleed air supply (66; 166; 266A, 266B) having a second pressure that is greater than said first pressure; and a valve (68; 268A, 268B) that, in use, selects between said first bleed air supply (64; 164; 264A, 264B) and said second bleed air supply (66; 166; 266A, 266B) to communicate said buffer supply air (62; 162; 262A, 262B) to said portion of the gas turbine engine; wherein the first bleed air supply (64; 164; 264A, 264B) is communicated as said buffer supply air (62; 162; 262A, 262B) in response to a high power condition of the gas turbine engine and said second bleed air supply (66; 166; 266A, 266B) is communicated as said buffer supply air (62; 162; 262A, 262B) in response to a low power condition of the gas turbine engine; characterised in that: the gas turbine engine, further comprises: a controller (70; 170; 270) that selectively modulates said valve (68; 268A, 268B) in response to the low or high power condition; and a sensor (99; 199) that detects said power condition; said portion is multiple bearing compartments of the gas turbine engine; and wherein said high power condition includes one of a take-off condition, a climb condition and a cruise condition, and said low power condition includes one of a ground condition, an idle condition and descent idle. a buffer system (60; 160; 260) that communicates a buffer supply air (62; 162; 262A, 262B) to a portion of the gas turbine engine, wherein said buffer system (60; 160; 260) includes: a first bleed air supply (64; 164; 264A, 264B) having a first pressure; a second bleed air supply (66; 166; 266A, 266B) having a second pressure that is greater than said first pressure; and a valve (68; 268A, 268B) that, in use, selects between said first bleed air supply (64; 164; 264A, 264B) and said second bleed air supply (66; 166; 266A, 266B) to communicate said buffer supply air (62; 162; 262A, 262B) to said portion of the gas turbine engine; wherein the first bleed air supply (64; 164; 264A, 264B) is communicated as said buffer supply air (62; 162; 262A, 262B) in response to a high power condition of the gas turbine engine and said second bleed air supply (66; 166; 266A, 266B) is communicated as said buffer supply air (62; 162; 262A, 262B) in response to a low power condition of the gas turbine engine; a first bleed air supply (64; 164; 264A, 264B) having a first pressure; a second bleed air supply (66; 166; 266A, 266B) having a second pressure that is greater than said first pressure; and a valve (68; 268A, 268B) that, in use, selects between said first bleed air supply (64; 164; 264A, 264B) and said second bleed air supply (66; 166; 266A, 266B) to communicate said buffer supply air (62; 162; 262A, 262B) to said portion of the gas turbine engine; wherein the first bleed air supply (64; 164; 264A, 264B) is communicated as said buffer supply air (62; 162; 262A, 262B) in response to a high power condition of the gas turbine engine and said second bleed air supply (66; 166; 266A, 266B) is communicated as said buffer supply air (62; 162; 262A, 262B) in response to a low power condition of the gas turbine engine; characterised in that: the gas turbine engine, further comprises: a controller (70; 170; 270) that selectively modulates said valve (68; 268A, 268B) in response to the low or high power condition; and a sensor (99; 199) that detects said power condition; said portion is multiple bearing compartments of the gas turbine engine; and wherein said high power condition includes one of a take-off condition, a climb condition and a cruise condition, and said low power condition includes one of a ground condition, an idle condition and descent idle. the gas turbine engine, further comprises: a controller (70; 170; 270) that selectively modulates said valve (68; 268A, 268B) in response to the low or high power condition; and a sensor (99; 199) that detects said power condition; said portion is multiple bearing compartments of the gas turbine engine; and wherein said high power condition includes one of a take-off condition, a climb condition and a cruise condition, and said low power condition includes one of a ground condition, an idle condition and descent idle.