William Matthew Sierra, better known by his ring name Bill Alfonso, is an American former professional wrestling referee and manager, currently performing for Major League Wrestling (MLW), where he serves as the manager of Matthew Justice. 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 professional wrestling, a referee is an authority figure present in or near the ring during matches. The referee's purpose is similar to that of referees in combat sports such as boxing or mixed martial arts, that is, as an arbiter of the rules and the person charged with rendering decisions. In reality, the referee is, like the wrestlers, a participant in executing a match in accordance with its script including its pre-determined outcome, and is responsible for controlling the flow of the match and for relaying information or instructions from backstage officials to the wrestlers. Like wrestlers, referees are also responsible for maintaining kayfabe, and must render decisions in accordance with the promotion's kayfabe rules.