William Edward Wagner, nicknamed "Billy the Kid", is an American former professional baseball pitcher who played 16 seasons in Major League Baseball (MLB). He pitched for the Houston Astros (1995–2003), Philadelphia Phillies (2004–2005), New York Mets (2006–2009), Boston Red Sox (2009), and Atlanta Braves (2010). Wagner is one of only eight major league relief pitchers to accumulate at least 400 career saves. A left-handed batter and thrower, Wagner stands 5 feet 10 inches (1.78 m) tall and weighs 180 pounds (82 kg).

In baseball, a save is credited to a pitcher who finishes a game for the winning team under certain prescribed circumstances. Most commonly a pitcher earns a save by entering in the ninth inning of a game in which his team is winning by three or fewer runs and finishing the game by pitching one inning without losing the lead. The number of saves or percentage of save opportunities successfully converted are oft-cited statistics of relief pitchers, particularly those in the closer role. The save statistic was created by journalist Jerome Holtzman in 1959 to "measure the effectiveness of relief pitchers" and was adopted as an official Major League Baseball (MLB) statistic in 1969. The save has been retroactively tabulated for pitchers before that date. Mariano Rivera is MLB's all-time leader in regular-season saves with 652, while Francisco Rodríguez earned the most saves in a single season with 62 in 2008.