This project was initiated to further develop the understanding of high temperature fatigue and creep-fatigue behaviour, both in the crack initiation/short crack growth and long crack growth regimes. It comprises in part a continuation of a post-doctoral project investigating microstructural effects on the creep-fatigue behaviour of U720Li. It builds upon the work already done in the post-doctoral project and further investigates the creep-fatigue behaviour of RR1000, a turbine disc superalloy recently developed by Rolls-Royce. Short crack and long crack tests at room and elevated temperatures were carried out on U720Li and its large grain (U720Li-LG) and large precipitate (U720Li-LP) variant as well as on the RR1000 material to complement the elevated temperature long crack data already obtained in the post-doctoral project. In the room temperature short crack tests, U720Li-LP exhibits the longest overall fatigue lifetime, followed by lower lifetime in RR1000, U720Li-LG and U720Li. The results of the room temperature long crack tests indicate approximately similar crack growth resistance of RR1000, U720Li and U720Li-LG while U720Li-LP shows lower crack growth resistance. Elevated temperature tests generally indicate better fatigue crack initiation and growth resistance in U720Li-LG and RR1000 compared to U720Li and U720Li-LP. Crack initiation and growth mechanisms under different testing conditions were identified and the complex interactions between microstructure and fatigue crack initiation and growth were examined through detailed testing and analysis. The difference in the trends of fatigue resistance of the materials under different testing conditions was attributed to the different effects of microstructure and/or alloy chemistry on fatigue mechanisms in the various fatigue crack initiation and growth regimes. Overall, larger grains and larger size and higher volume fraction of coherent γ' precipitates were identified to be microstructural characteristics necessary for improved fatigue performance of turbine discs. Large grains have been shown to be beneficial under most of the different test conditions conducted in the current study. In terms of the size and volume fraction of coherent γ' precipitates, generally higher crack growth resistance was noted for larger size and higher volume fraction of coherent γ' precipitates in the elevated temperature long crack tests conducted in air. Although turbine disc fatigue life has been known to be dominated by crack initiation and short crack growth at elevated temperatures, higher crack growth resistance in elevated temperature long crack tests in air which has been shown to correlate to improved high temperature short crack growth resistance will give increased confidence in the overall fatigue performance of the turbine disc alloy. Porosity control is also important as porosity was noted to be the predominant crack initiation mechanism at both room temperature and at 650°C.