A nickel base superalloy comprising 14.0 to 20.0 wt % cobalt, 13.5 to 17.0 wt % chromium, 2.5 to 4.0 wt % aluminium, 3.4 to 5.0 wt % titanium, 0 to 3.0 wt % tantalum, 3.8 to 5.5 wt % molybdenum, 0.035 to 0.07 wt % carbon, 0.01 to 0.04 wt % boron, 0.055 to 0.075 wt % zirconium, 0 to 0.4 wt % hafnium and the balance nickel plus incidental impurities. The nickel base superalloy is suitable for use as gas turbine engine high pressure compressor rotor discs or turbine discs. It is capable of operation at temperatures above 700.degree. C. and has good fatigue crack propagation resistance, creep resistance and tensile strength.