After describing urine testing technology and the way it is currently used with probationers, the article provides an overview of probation law and descriptions of three legal models of probation urinalysis. The discussion then turns to the guarantees of the fourth amendment and includes an analysis of how fourth amendment principles apply to probation searches and seizures in general and urinalysis in particular. The analysis concludes that although urinalysis based on reasonable individualized suspicion is constitutional, the legal status of suspicionless drug testing is less certain. The article then considers how best to accommodate fourth amendment concerns in probation urinalysis while giving due deference to the government's compelling interest in using urine testing to promote rehabilitation and protect the public. The article proposes that suspicionless testing be limited to offenders who are, on the basis of objective facts, reasonably believed to be drug users. This would be done on occasions determined by nondiscretionary criteria. 413 footnotes