On Monday the Supreme Court ruled unanimously that police violated the 4th amendment when they placed a Global Positioning System (GPS) tracking device on a suspect’s car and monitored its movements for 28 days. In his opinion on the case, Justice Anthony Scalia wrote that the use of GPS constituted a “search” and therefore requires a warrant. This ruling may have an impact on other cases where GPS was used, as well as other types of surveillance mechanisms. Jeffrey Rosen is a professor of law at George Washington University. Attorney Marc Rotenberg is director of the Electronic Privacy Information Center, or EPIC.