Three out of four arrestees in the Netherlands were never involved in crime before they reached adulthood. However, the general assumption in criminological research is that crime emerges early in life. Prevention and intervention are therefore mainly focused on youths. Consequently, we know the least on the largest group of offenders: those with an adult crime onset. The processes that cause juveniles and adolescents to offend are well studied but we hardly have data or theories on these processes for offenders who initiate criminal behaviour in adulthood. However, there is substantial evidence that adult onset is more common than currently assumed and therefore research should focus on its explanations and crime prevention strategies should refocus their efforts in order to reduce crime. The aim of my proposed research is threefold. Firstly, I will reliably assess the size of the population of adult-onset offenders. Secondly, I will characterize both adult-onset offenders as well as their criminal careers. Thirdly, I will employ a multi-method approach in which I both test the impact of general risk factors on adult criminal career development, as well as seek novel explanations for adult-onset offending through interviews with adult-onset offenders themselves. I have access to both quantitative population based prosecution data, as well as an existing data source that captures criminal careers for a large representative sample of offenders, that I will supplement with interview data. This will provide a unique and sound empirical base for the proposed study. The findings will be made available to policy makers and practitioners through a national workshop with police officers, the prosecution service and the probation service, in which their practical use for both prevention and intervention in adult-onset will be outlined.