For economic and environmental reasons, there is a drive towards improving the efficiency of civil aircraft gas turbines, which can be done by increasing their operating temperature. This requires new or improved materials; however the behaviour of these at elevated temperatures must be understood to avoid catastrophic in-service failure. Nickel-based superalloys are typically used for disc applications in gas turbines. These discs are designed using fracture mechanics and defect tolerance design approaches. By conducting crack growth tests at high temperatures using nickel superalloy testpieces, crack growth rate and behaviour can be obtained and a service life can be defined. However, crack growth during these tests may either be continuous or may decelerate, and this must be considered for accurate service life prediction. Testpiece geometry, addition of side-grooves, or microstructure variation may also affect crack growth rate. This project focuses on the crack growth behaviour of two Ni-based superalloys, used in turbine discs of Rolls-Royce gas turbines. One alloy is currently in service, the other is still under development. Experiments simulating gas turbine operating conditions have been conducted to understand how cracks grow in these materials. The effect of testpiece geometry (considering both corner-initiating cracks and through-thickness cracks) on crack growth rate has been investigated, through the use of compact tension (CT), single edge notched bend (SENB), and corner crack (CC) testpieces. In addition, the effect of side-grooves in SENB testpieces on crack growth rate has been studied. By developing a better understanding of crack growth in these alloys, an accurate design life can be determined.