Unlike conventional search warrants, which usually require delivery of evidence or contraband to a particular location before police officers can search the premises, anticipatory or prospective search warrants allow the officer to seek permission in advance based on "probable cause" that evidence will be found at the location at the time the warrant is actually executed. While the Supreme Court has never directly addressed the issue of anticipatory warrants, numerous lower courts have ruled that it is constitutionally permissible to obtain such a warrant. Most challenges to the constitutionality of these warrants revolve around claims that the fourth amendment probable cause requirement has not been satisfied. For purposes of studying case histories, court decisions involving various investigative applications of anticipatory warrants can be grouped into three categories, according to the degree of police control over the delivery of evidence: mail deliveries, controlled deliveries by cooperating witnesses, and delivery uncontrolled by the government. Apropos of this, some courts have required that officers prove that the contraband or evidence is on a "sure course" to its destination before the warrant is issued. Perhaps the greatest advantage of the anticipatory warrant is the speed with which it can be secured, offering a practical alternative to proceeding without a warrant and thereby running the risk that evidence will be suppressed in court. 19 footnotes.