The transition from localised corrosion to environmentally assisted cracking (EAC) is a phenomenon that remains poorly understood despite its importance to many industrial applications. Although the transition stage is often noted, very few detailed studies have been performed to quantify it. Some authors believe that the local environment is the fundamental contributor to EAC initiation in structural materials, while others consider the geometrical discontinuity introduced by a localised corrosion site such as a pit or crevice to be the significant factor. Assessment of the transition stage is often based on the competing growth rate concept originally proposed by Kondo [1], where it is assumed that crack initiation occurs at the base of a corrosion pit when; (i) the pit achieves a critical depth and hence critical stress intensity for crack initiation, and (ii) the crack growth rate exceeds the pit growth rate. This project aims to develop techniques and methodologies of measure that can be used to provide a more meaningful analysis of this transition effect. Two independent investigations of the transition phenomena have been presented which rely on the emerging field of X-ray tomography that can be used to study localised corrosion and environmentally assisted cracking events. InPart A of this dissertation, real time intergranular corrosion and stress corrosion crack growth rates have been quantified via synchrotron X-ray tomography in sensitised high carbon (0.056 wt.% C) type 304 stainless steel exposed to dilute potassium tetrathionate solutions. Crack growth rates, in both the long and short crack regimes, have also been measured using traditional fracture mechanics based tests. The transition from localised corrosion to EAC is discussed on the basis of Kondo's competing growth rate model. Results illustrate the unique power of the X-ray tomography technique in acquiring growth rates for localised corrosion and environmentally assisted cracking. In Part B of this dissertation, focus tube X-ray tomography is utilised for studying the threedimensional evolution of pitting and stress corrosion cracking in 3NiCrMo V steam turbine disc steel. Results indicate that pit and crack depth are not necessarily the same at the point of transition, and that cracks nucleate more readily at the pit side, just below the sample surface, in contrast with the assumption of existing deterministic (mechanistically based) models [2-6] developed to address SCC initiation and propagation in low-pressure steam turbines. With this in mind it is suggested that the existing pit-to-crack models based on Kondo's criteria should be reassessed and modified to incorporate empirical aspects specific to the system of interest.