This thesis describes the work undertaken over three years into the investigation of the effect of proton irradiation on the fatigue crack behaviour of an austenitic stainless steel. Many in-core nuclear reactor components that are subjected to radiation damage and repeated thermal stresses are made from austenitic stainless steels. A thorough understanding of the effect of the material changes caused by the radiation damage on the fatigue cracking behaviour is essential for safely extending the lifetime of nuclear plant components. Proton irradiation from an ion accelerator has been used in this study as a surrogate for the neutron irradiation experienced by real components. Despite the different interactions of protons and neutrons with matter, under certain conditions, proton damage can result in a similar damaged end-state to neutrons. In contrast to reactor neutrons, accelerated protons can be produced at a higher rate, allowing component end-of-life damage levels to be reached in a much shorter time. A review of the literature identified thermoelastic stress analysis (TSA) as the technique most capable of measuring the required variables during a fatigue crack growth test, i.e. stress intensity factor and plastic zone size. Two key gaps in the literature were identified. Firstly, few studies of fatigue crack growth in irradiation damaged specimens exist and those that do typically report a small number of specimens. Due to the inherent variability of fatigue crack growth, a large number of specimens should be tested at each damage level to obtain statistically meaningful data. Secondly, fatigue crack growth studies rely on analytical relations based on crack length for both stress intensity factor and plastic zone size measurements in many cases. Using TSA allows direct measurement of these quantities and hence a deeper understanding of the actual specimen behaviour. A novel method was developed to measure the extent of the cyclic plastic zone based on the in-phase second harmonic temperature signal. This allowed measurement of the plastic zone area, without prior knowledge of the yield strength. Further, the total energy dissipated from the plastic zone could be calculated, without assuming the plastic zone size and shape. Investigations of the second harmonic temperature data revealed signals originating in the crack flanks that were demonstrated to be related to crack closure. This allowed a simple binary check for closure without further data analysis. A number of austenitic compact tension specimens were prepared and irradiated using accelerated protons at a set of increasing proton fluence levels. Following irradiation, the specimens were loaded in fatigue and imaged regularly with a TSA system as the crack grew. This gave simultaneous measurements of plastic zone size and effective stress intensity factor, and represented the first time the TSA technique has been used to investigate radiation damaged material. The experimentally obtained stress intensity factors and crack growth rates were fitted using the Paris' Law model and analysis of covariance suggested that there is not a statistically significant difference in the gradient of the fit with increasing irradiation damage. However, a significant effect (p < 0.05) was found in the offset value. This suggests that greater radiation damage causes a greater reduction in crack growth rate, in agreement with literature. Measurements of the TSA output suggest that, contrary to theory, the apparent plastic zone increased in size with more radiation damage. However, the dissipated energy from the plastic zone did tend to increase with radiation damage. Hence, it is hypothesised that the physical plastic zone decreases in size due to the greater hardness induced by the irradiation, but emits a larger amount of energy as the vacancies and interstitials generated by the radiation damage make dislocation movement more energetically difficult, resulting in an apparent increase in plastic zone size when measured using the TSA system.