The Reasonable Expectation of Privacy Test is one of the most prominent judicial doctrines commonly used when reviewing government law enforcement under the Fourth Amendment to the United States Constitution. There are pros and cons between scholars and judges about this test and The Third-party Doctrine derived from this test. This article focuses on the government's electronic surveillance activities during the law enforcement process, through analysis of Electronic Communications Privacy Act and relevant case law, sorts out and summaries the latest development of U.S. jurisprudence, theories and legislation, and seeks implications for Japanese law.