William Matthew Sierra is an American former professional wrestling referee and manager better known by his ring name Bill Alfonso. He achieved his greatest success in Extreme Championship Wrestling in the mid-to-late 1990s. He is well known for the whistle that was almost always hanging around his neck, which he blew constantly during his wrestlers' matches In May 1995, he made his debut in Extreme Championship Wrestling as a "trouble-shooting referee".Introduced by Shane Douglas, he angered many ECW fans with strict enforcement of the rules and disqualifications of wrestlers for such ECW traditions as using steel chairs as weapons.n this role, he engaged in a feud with ECW founder Tod Gordon that resulted in several wrestling matches.

Since the closing of ECW, Alfonso has appeared in several independent promotions. He worked one match in Xtreme Pro Wrestling as a referee. He has also worked in Elite Wrestling Entertainment as a referee and a manager,as well as managing wrestlers in Squared Circle Wrestling and Pro Wrestling Unplugged.In May 2007, he made his first appearance with Women's Extreme Wrestling, where he serves as a manager and as commissioner.

On August 8, 2010, Alfonso took part in Total Nonstop Action Wrestling's ECW reunion show, Hardcore Justice, managing both Rob Van Dam and Sabu during their match against each other. On the following edition of TNA Impact!, the ECW alumni, known collectively as Extreme, Version 2.0 (EV 2.0), were assaulted by A.J. Styles, Kazarian, Robert Roode, James Storm, Douglas Williams and Matt Morgan of Ric Flair's Fourtune stable, who thought they didn't deserve to be in TNA.

In October 2002, fellow manager Missy Hyatt sued Alfonso in a case televised on Judge Mathis. She accused Alfonso of damaging her Porsche and was awarded $500.

Alfonso is not the only person in his family to have a career in the Professional Wrestling sport, his step brother, former Olympian Bob Roop also spent many years as a wrestler fighting all over the world under a variety of names and gimmicks while his cousin David Sierra has also enjoyed a long career in the ring including a stint in WCW as Fidel Sierra where he is remembered for a controversial feud with Hacksaw Jim Duggan. However, contrary to popular belief, Alfonso is not related to Mike Awesome with whom he spent time with in ECW. The confusion is thought to have stemmed from the fact that Awesome's real surname was Alfonso and the fact that he wrestled in Japan as The Gladiator, a name that Alfonso's real brother Bob Roop also used during his career.