Drug interdiction is carried out primarily by the Coast Guard, the Customs Service, and the Immigration and Naturalization Service. Despite increasing expenditures in support of drug interdiction, the U.S. Department of Defense remains a support agency whose activities can enhance those of the primary interdiction agencies but cannot substitute for them. Military efforts are constrained by legal restrictions, and interdiction is a secondary activity of military units. Merely increasing the capability to detect and identify more potential targets through patrol is not likely to increase the number of seizures made to any great extent. Moreover, increased interdiction efforts are not likely to have much effect on cocaine availability because only a small share of total drug distribution costs are accounted for by smuggling. Unless interdiction can substantially increase the costs of smuggling, more effective interdiction will have only modest effects on cocaine consumption. For marijuana, interdiction has clearly had an impact on the level of imports from countries other than Mexico. Using the military to increase the probability of detection of seaborne marijuana might further raise the price of the drug. However, the vulnerability of the Mexican border, the strength of domestic production, and the potency of recent crops may greatly mitigate the effects of improved maritime interdiction. Appendixes include additional information on the effects of interdiction on drug export and prices. Figures, tables, and 39 references.