Stemming the flood of illegal aliens into the United States is a major policy issue for the US Government and its enforcement arm, the Immigration and Naturalization Service (INS). There is also a heavy impact on local and State law enforcement agencies, which cooperate with the INS to carry out investigations, make arrests, prepare case reports, prosecute cases, house criminal aliens and finally deport them. In California alone, 16 percent of the inmates in the total prison population are criminal aliens. The INS is divided into two main functions: examination and enforcement. The Service has an interdiction arm, an investigations arm, and a detention arm. There are 1,600 INS agents for the entire country, which necessitates good cooperation with local, State and other Federal law enforcement agencies. The war on drugs and the war on violence are two major areas where INS is actively involved with other agencies. Ideally, FBI, DEA or local law enforcement capture and incarcerate illegal, criminal aliens and INS can track them through the system to identify those eligible for deportation. Various elements throughout the criminal justice system are working to develop information-sharing procedures that will facilitate the total process. This article describes some of the successes, failures, and difficulties of the effort.