This chapter introduces the available research on long-term crime trends and shows the major role that cross-national comparisons have played since the nineteenth century, revealing that the first researchers were fully aware of such comparisons’ pitfalls. Since then, most research has focused on homicide trends—often used as a proxy for trends in all violent crime—in industrialized Western nations, because data are not widely available for other regions of the world and other types of offenses. Problems related to finding appropriate crime measures and valid denominators for the absolute numbers provided by such measures are also investigated, as are the risks of chronocentrism, parochialism, and causationism. Finally, it cautions that explaining the decrease in Western industrialized countries’ interpersonal violence since the late Middle Ages through the effect of a civilizing process requires overlooking the genocides, mass killings, and war crimes committed in these countries and their former colonies.