Abbott and Costello first aired the sketch called "Who's on First?" for The Kate Smith Hour radio program in March 1938. It may have been written by burlesque producer John Grant and Will Glickman, who had taken the idea from previous sketches like "The Baker Scene" and Who Dyed". Bud Abbott later admitted that it may have been taken from an older routine called "Who's The Boss?". From the skit:

Position Player

First base Who

Second base What

Third base I Don't Know

Left field Why

Center field Because

Pitcher Tomorrow

Catcher Today

Shortstop I Don't Give a Darn

Abbott and Costello started their vault to stardom from a vaudeville revue called "Hollywood Bandwagon" and continued to radio, TV, and finally the Big Screen.

Though scripted, the duet rarely performed the act "rehearsed", playing on spontaneity rather than controlled lines. Bud Abbott had a way of reading an audience to allow improv to play an important part of their sketches.

Time named the sketch as the "Best Comedy Sketch of the 20th Century".

The Library of Congress holds a recording for prosperity and the American Film Institute has included the line "Who's on First?" as one of the 100 most memorable quotes in film making history.

More Info: en.wikipedia.org