Hydrogen attack is a degradation phenomenon that affects process equipment operated at elevated temperatures in an environment containing a high hydrogen partial pressure. It has been the subject of numerous studies over the years prompted by damage discovered during routine inspections, or incidents that have occurred in service. As non-destructive evaluation (NDE) techniques have improved, damage is being detected during earlier stages where safe operation may still be possible for some time period. This work focuses on the fitness for service evaluation of equipment containing high temperature hydrogen attack (HTHA) using a continuum damage mechanics (CDM) approach. The model can be employed to assess the loss in load bearing capacity due to damage in the form of widespread micro-fissuring and voids (i.e. up to the point of macro-crack coalescence). Experimental data from literature sources have been used to develop a relationship between damage rate and operational loading conditions. The predictions are compared to field experience to illustrate key aspects of this approach.