This chapter introduces the circumstances under which criminal justice professionals have legal authority to stop, question, detain, and arrest people in society. The chapter begins with a primer on how to analyze Fourth Amendment problems (pp. 194–196), discusses how the U.S. Supreme Court recognizes a reasonable expectation of privacy in the U.S. v. Katz (1967) case (pp. 196–197), and continues the exploration started in Chapter 1 of the Riley v. CA (2014) case examining how the Fourth Amendment has had to adapt to privacy interests defined by modern technology (pp. 198–202), including pen registers, StingRays, and dirtboxes culminating in a narrow definition of the third-party doctrine announced in Carpenter v. US (2018). The chapter also examines the Terry stop, including duration, requests for identification, in automobiles, and the intersection of the Terry stop and race (pp. 203–207). Arrest and the Fourth Amendment requirements for use of force are explored (pp. 207–210). The requirements for a warrant and the steps in execution are explained, including sources of probable cause with an excerpt from the search warrant of the BALCO steroid distribution investigation used for illustration (pp. 210–221). Making the Courtroom Connection examines the legal intricacies of establishing probable cause and the defendant’s challenge through a Franks hearing (p. 214). The chapter ends with a discussion of the service of the warrant; knock and announce, detaining people on the premises during the warrant’s execution, and the return (pp. 220–222).