A Sign is a form of non-verbal communication used to convey messages that must remain hidden to the opposite team. The most common signs include those made by the catcher by using his fingers to signal to the pitcher what pitch he wants him to throw, and those made by the coaches telling the batter and baserunners if a specific strategy is called for, such as the bunt, the hit-and-run, the stolen base, etc.
Because there is a great tactical advantage in knowing what pitch is coming or what strategy is being called for, the signs are given in a code that is increasingly sophisticated as the situation becomes more crucial. There is an art to "stealing signs", i.e. decoding the opposite team's signs in order to interpret what messages it is passing along to its players. This is an accepted part of baseball as long as it is done using contemporaneous observation and brainpower; however, the recourse to technology is to do so, such as television cameras, is considered cheating. in 2019, Major League Baseball tightened its rules in this area, following allegations of violations in previous years, with penalties that could could go all the way to the forfeiture of draft picks or international bonus pool money.
The first team to be caught under the more draconian policy were the Houston Astros, who had implemented a system using technology to steal their opponents' signs in 2017, on their way to winning the World Series. On January 13, 2020, General Manager Jeff Luhnow and manager A.J. Hinch were both given one-year suspensions and the team was fined $5 million and had to forfeit their top two draft picks in the next two amateur drafts. Hinch and Luhnow were later both fired by the Astros after the suspension announcements. The Boston Red Sox were later punished as well, albeit more lightly, for a similar scheme in 2018, a year in which they also won the World Series. The Sox's manager, Alex Cora, who was also suspended for a year, had been the Astros' bench coach the previous season.
It is claimed that the first catcher to give signs to the pitcher was Scott Hastings.
One sign for which secrecy is not an issue is the stop sign.
- Paul Dickson: The Hidden Language of Baseball: How Signs and Sign Stealing Have Influenced the Course of our National Pastime, Walker & Company, New York, NY, 2003. ISBN 978-0802777195
- Gabe Lacques: "Severe punishment looms for Houston Astros in sign-stealing scandal", USA Today, January 10, 2020. 
- Gabe Lacques: "Injured parties, defiant execs and a tainted title: Houston Astros' sign-stealing scandal checks all the boxes", USA Today, January 15, 2020. 
- Andy Martino: Cheated: The Inside Story of the Astros Scandal and a Colorful History of Sign Stealing, Doubleday Books, New York, NY, 2021. ISBN 978-0385546799
- Bob Nightengale: "After Red Sox's sign-stealing, MLB needs to curb the game's growing tech sector", USA Today Sports, September 5, 2017.