This paper illustrates how to estimate criminal justice system costs by offence type. Criminal justice system costs are all the costs the authorities incur to prevent and investigate crime, prosecute criminals, impose sentences, and take care of victims and offenders. There are two approaches: the break-down and the bottom-up approaches. The break-down approach decomposes the aggregate budget into smaller pieces. The bottom-up approach multiplies known costs per activity by volumes for each activity and offence type. Both approaches can be combined with two types of estimates: incidence-based and prevalence-based estimates. An incidence-based estimate identifies all costs attributable to crimes committed or processed in a specified period regardless of whether these costs exceed this time period. A prevalence-based estimate identifies all costs incurred in a specified period regardless of when the crime was committed or processed. This paper looks at the differences between the two approaches and the two estimates and indicates which combination works best depending on the type of analysis and the availability of data. The methodologies are illustrated using examples from The Netherlands. These examples show that the availability of reliable data is crucial. The more assumptions have to be made, the less reliable the end results. Investing in better data in this area should be a first priority for governments interested in criminal policy evaluation. Tables and references (Published Abstract)