This paper contains the law professors' brief in the landmark case of Warshak v. United States, the first federal appellate case to recognize a reasonable expectation of privacy in electronic mail stored with an Internet Service Provider (ISP). While the 6th circuit's opinion was subsequently vacated and reheard en banc, the panel decision will remain extremely significant for its requirement that law enforcement agents must generally acquire a warrant before compelling an ISP to disclose its subscriber's stored e-mails. The law professors' brief, co-authored by Susan Freiwald (University of San Francisco) and Patricia L. Bellia (Notre Dame) and signed by fifteen professors of electronic privacy and internet law, argues that stored e-mail should be protected by the Fourth Amendment warrant requirement notwithstanding the fact that it is accessible from third party ISPs. The brief refutes the government's argument that its agents must satisfy only a weak subpoena standard before compelling an ISP to disclose stored e-mails, and argues against a broad "third party rule." The 6th Circuit panel expressly referred to the law professors' brief (along with an amicus brief by civil liberties groups) when it held that stored e-mails are properly analogized to telephone calls. It found that when agents conduct electronic surveillance, the same Fourth Amendment protection available under Katz v. United States to telephone calls in transit should extend to e-mails in storage. In addition to containing the brief submitted to the 6th circuit panel, this paper provides legal and factual background to the Warshak case.