Understanding of the phenomenon of crime lies at the heart of criminology. A century and a half of theory and research has accumulated, but there does not yet exist an evaluation of how much explanatory power (summarized as the amount of variance explained) there is in criminological research. Examination of empirical tests of criminological theory in Criminology between 1968 and 2005 yields three key findings. The overall level of variance explained is often very low with 80 or 90 percent unexplained. There has been no improvement over time. Individual-based models provide relatively weak explanatory power, but models that took a more crime-specific focus indicated some strength. Criminologists will need to pay much more attention to "what is not explained" in criminological modeling if they are to make significant advances in understanding crime.