The different steps of stress corrosion have been described in the previous chapter. We will focus here on what is identified as the steps of rapid propagation, coalescence and ramification. In the first case, it involves studying the growth of a single crack propagating under the simultaneous effects of the environment and mechanical loads. Knowing the growth kinetics of these defects allows us to estimate and/or model the residual life of components on which they have been detected by non-destructive techniques. In contrast to initiation for which there are several definitions, the cracks concerned by the so called rapid propagation step are generally several millimeters in length. Many testing methodologies allow the growth of a network of cracks and therefore take an interest in the phenomena of coalescence and branching.