Several key parameters were introduced by criminal career researchers to better understand the longitudinal sequence of offenses committed by individuals. Two of the four key parameters defined and described by criminal career researchers are: participation and frequency. Criminal career researchers posited that participation and frequency are two distinct phenomena that may be driven by different motives and explained by different causal mechanisms. This chapter draws on recent advances in capture-recapture modelling to estimate the size of sexual aggressor population. Doing so also allows to estimate the risks of incarceration for sex offenders in Quebec. The data consist of a sample of 387 individuals convicted for serious sexual offense between January 1995 and June 1998 in Quebec. Social scientists have pursued different avenues to establish the extent and nature of problem of sexual aggression, and three main strategies have been chosen over the years: victimization surveys, self-report studies, and official statistics.