John Alec Entwistle (October 9, 1944 - June 27, 2002) was an English bass guitarist, songwriter, singer, and horn player, who was best known as the bass guitarist for the rock band The Who. He is regarded as one of the greatest and most influential rock bassists of all time, creating an aggressive lead sound that helped influence contemporary and later bassists such as Steve Harris, Lemmy, Geddy Lee, John Paul Jones, Phil Lesh, Noel Redding, Billy Sheehan, Chris Squire, Duff McKagan, Markus Grosskopf, Mike Watt, Peter Hook and Krist Novoselic.

Entwistle helped uncover the potential of the bass guitar as a lead instrument, using aggressive pentatonic lead lines, and a trebly sound virtually unheard of in the early 1960s. He pioneered the use of roundwound steel bass strings, made to his personal specifications by RotoSound. His search for a sound to cut through The Who's sonic onslaught led him to experiment with more and different basses, leading him to amass a collection of over 200 instruments by the time of his death. His search for the perfect sound led him to experiment most notably with Fender and Rickenbacker basses in the 1960s, Alembic's basses in the 1970s, Warwick in the 1980s, and Status all-graphite basses in the 1990s.