This article explores the Fourth Amendment issues to be considered when establishing a transportation facility drug interdiction unit. The fact-specific nature of each transportation interdiction situation often requires quick, on-the-spot judgments concerning unique applications of Fourth Amendment principles. Cases decided by the Supreme Court since 1980 concerning interdiction at transportation facilities are summarized briefly. Two cases, United States v. O'Neal and United States v. Buenos, are considered in detail to emphasize the importance that officers be able to discern the intricacies of applying the Fourth Amendment to rapidly developing situations during brief encounters at such facilities. The author stresses that local prosecutors and law enforcement agency counsel should be consulted and police personnel working in transportation facility interdiction units should be provided with training specifically directed to include Fourth Amendment issues. Footnotes