Drug availability estimates for 2001 were developed for the four major drugs: cocaine, heroin, methamphetamine, and marijuana. This effort responded to a request from the U.S. Attorney General for measurement of the quantity of illicit substances available to drug users in the United States. There is significant uncertainty in these estimates due to the illicit and clandestine nature of the various drugs, and the limited data currently collected to aid in these analyses. Therefore, caution is urged in the application of these estimates. Six interagency working groups (one for each of the drug-types, a consumption working group and a seizure working group) were formed and staffed by members of the following federal agencies: Crime and Narcotics Center, U.S. Customs, Drug Enforcement Administration, Defense Intelligence Agency, Department of Justice, National Institute of Justice, El Paso Intelligence Center, Joint Interagency Task Force West, National Drug Intelligence Center, Office of National Drug Control Policy, Substance Abuse and Mental Health Services Administration, Department of Treasury, U.S. Coast Guard, and the U.S. Interdiction Coordinator. This document is the compendium of the reports from each of the six working groups.