Escalation in crime seriousness over the criminal lifecourse continues to be an important issue to study in criminal careers. Quantitative research in this area has not yet been well developed owing to the difficulty of measuring crime seriousness and the complexity of escalation trajectories. In this paper we suggest that there are two types of escalation process-escalation associated with experience of the criminal justice process, and escalation associated with age and maturation. Using the 1953 birth cohort from the England and Wales Offenders Index followed up to 1999, and a recently developed seriousness scale of offenses, we constructed the individual sequences of seriousness scores from conviction to conviction. These individual sequences were then analyzed using a variety of longitudinal mixed models, with age, number of conviction occasions, sex and number of offenses used as covariates. The results suggest that ageing is associated with de-escalation whereas the number of conviction occasions are associated with escalation, with the two processes pulling in different directions. This conceptual framework helps to disentangle previously contradictory results in the escalation literature.