William Joseph Klem, born William Joseph Klimm (February 22 1874 – September 16 1951), known as the "father of baseball umpires", was a National League umpire in Major League Baseball from 1905 to 1941. He had the longest career of any major league umpire (37 years) before Bruce Froemming tied that mark in , and was also the oldest umpire in history at age 67 until Froemming surpassed that mark as well. Klem was widely respected for bringing dignity and professionalism to umpiring, as well as for his high skill and good judgment. Klem was also an innovative umpire; he was the first major league umpire to use arm signals while working behind home plate, and was one of the first umpires to wear a modern, somewhat pliable chest protector inside of the shirt – a move which he successfully campaigned to have adopted throughout the NL. He was the first to straddle foul lines for better perspective and stand to the side of the catcher for a better look at pitches. Finally, he was the last umpire to routinely work the plate in all games (traditionally the crew chief always worked the plate; today umpire crews rotate base/plate assignments).
Born in Rochester, New York, he worked a record 18 World Series: 1908, 1909, 1911, 1912, 1913, 1914, 1915, 1917, 1918, 1920, 1922, 1924, 1926, 1929, 1931, 1932, 1934 and 1940. No other umpire has worked in more than ten Series. Of the 16 major league teams in existence during his career, all but one – the St. Louis Browns, who would not win a pennant until 1944 – appeared in a World Series that he officiated; the only other teams which did not win a championship with Klem on the field were the Brooklyn Dodgers and Philadelphia Phillies, neither of which won a title during Klem's lifetime, and the Detroit Tigers. He was also one of the umpires for the first All-Star Game in 1933, and worked behind the plate for the second half of the game; he later umpired in the 1938 All-Star Game as well.
He called balls and strikes in five no-hitters, an NL record later tied by Harry Wendelstedt. He was also the home plate umpire on September 16, , when Jim Bottomley of the St. Louis Cardinals had a record 12 runs batted in. Klem had a number of nicknames amongst the players: his favorite was "The Old Arbitrator", but his jowly appearance also led to some players calling him "Catfish". Klem despised the latter name, and was notorious for ejecting players whom he caught using it. One particular incident involved a player who Klem ejected after he caught the player drawing a picture of a catfish with his foot in the infield dirt.
He also dismissed catcher Al Lopez from a game after Lopez pasted, onto home plate, a photo he clipped from a newspaper, which showed Klem clearly in error calling a play involving Lopez. The catcher had covered the photo with dirt and waited for Klem to brush off home plate (Source: Baseball's Greatest Managers, 1961.).
Klem died at age 77 in Miami, Florida. He and Tommy Connolly were the first two umpires inducted into the Baseball Hall of Fame in . They are also the only two umpires to have worked in five different decades.