Despite the increase in research on the police, U.S. police agencies in general have been the subject of limited scientific study. Recent research interest in the police stems from greater police involvement in major social and political events. Existing police studies tend to explore topics with immediate policy implications. State police represent an integral part of the U.S. police system. Yet, unlike municipal police, State police forces have received no attention from police researchers and historians. State police research offers an opportunity to study law enforcement agencies that were specifically intended to operate in rural versus urban settings. The author historically reviews State police development and operations by looking at available literature and discussing early efforts to centralize the police in Ireland and Canada. He examines the emergence of State police in the United States in relation to social and historical developments. Particular attention is paid to the creation of State police agencies in Illinois and Colorado. The author concludes that State police forces resulted from the progressive reform movement, with primary motivating factors being the increase in immigration from eastern and southern Europe and the desire to curb the growing labor movement. An appendix suggests areas for further research on State police. References, notes, and tables