Variational phase field fracture models have received significant attention in the last decade. Building upon Griffith’s energy balance and the thermodynamics of fracture, phase field models have enabled the prediction of complex cracking features such as nucleation from multiple sites or the coalescence of numerous defects, in arbitrary geometries and dimensions. Recently, the framework has been extended to capture fatigue damage, introducing a cyclic degradation of the fracture energy that naturally captures Paris law behaviour and stress-fatigue life (S-N). In this work, we extend existing phase fatigue models by defining a new damage accumulation definition that accounts for: (i) arbitrary slopes of the S–N curve, (ii) the fatigue endurance limit, and (iii) the mean stress effect. The comparison with experiments demonstrates that the model can reliably predict fatigue lives and endurance limits, as well as naturally capture the influence of the stress concentration factor and the load ratio, without the need for fitting [1]. Moreover, we extend phase field fatigue to the analysis of hydrogen-assisted fatigue crack growth, a problem of notable technological importance. A multi-field framework is presented whereby the hydrogen concentration is solved for and the fracture energy is further degraded with hydrogen content through a mechanistic, implicitly multi-scale approach. We show that the modelling framework presented can be used to predict the impact of the environment on fatigue crack growth rate curves and S-N curves [2], enabling optimising design and maintenance through Virtual Testing, as well as planning efficient and targeted experimental campaigns.