In the narrow 5-3 decision of Smith v. Maryland (1979), the Supreme Court warranted perhaps one of the greatest intrusions upon privacy with the establishment of the Third Party doctrine. The defendant, Michael Smith, was found guilty of robbing and sending threatening phone calls to Patricia McDonough. However, the controversy in this case lies not in Smith’s innocence or guilt, but rather in the legality of how the incriminating evidence was obtained: the warrantless use of pen registers. In this case, pen registers were used to record all numbers dialed from Smith’s phone, tracing the threatening phone call back to him.[1]

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