The work notes that computational criminology applies computer simulations to study topics of interest for criminologists, and that the validity of the simulations ultimately depends on whether they are able to reproduce empirical phenomena with sufficient accuracy. The sole means cited to determine this is by comparing model output to real observations. This paper provides an overview of how such model evaluations can be undertaken. The work provides background on the types of credibility, citing qualitative, internal qualitative, and external qualitative, along with a detailed discussion of data-based computer model evaluation. These include: model specification; determining evaluation criteria; data collection; constructing model approximations; analyzing model output; and revision of the model. The authors concluded that there is no substitute for testing a computer model against data, as it is a well-understood and widely accepted principle in the scientific disciplines that make extensive use of simulations. It is recommended that criminology settle for nothing less. Figure and references