There has been a recent increase in the level of drug trafficking-related violence within and between the drug trafficking organizations in Mexico. This violence has generated concern among U.S. policy makers that the violence in Mexico might spill over into the U.S. Currently, U.S. federal officials deny that the recent increase in drug trafficking-related violence in Mexico has resulted in a spillover into the U.S., but they acknowledge that the prospect is a serious concern. Contents of this report: (1) Intro.; (2) The Southwest Border Region and the Illicit Drug Trade Between the U.S. and Mexico; (3) Relationship Between Illicit Drug Markets and Violence; (4) What is Spillover Violence?: (5) Challenges in Evaluating and Responding to Spillover Violence.