Karl Matthew Alvarez (born March 10, 1964) is the bass guitar player for both the Descendents and All, the band that resulted after the Descendents disbanded again in 1987. Alvarez joined the Descendents after the Enjoy! album from his previous band The Massacre Guys, and played on all of the All albums, and the Descendents albums All, Everything Sucks and Cool To Be You. He plays finger style bass and provides backing vocals when live (and lead vocals as heard in "Cause" on the All Live Plus One album). Since joining the band he has been a major songwriter contributing many songs to All (both the album and the band), Everything Sucks and Cool To Be You. In the summer of 2006 he joined Gypsy Punk band Gogol Bordello for part of the Van's Warped Tour and the Reading and Leeds Festivals. Karl currently plays guitar and sings in Underminer and the Vultures.
Since 2004 Karl has played with The Last along with Descendents/All drummer Bill Stevenson. He is currently a singer in a band called Underminer.
In 2006 Karl played on The Lemonheads self-titled comeback album which was released on Los Angeles' Vagrant Records.
During their last two Canadian tours (2007 and 2009, respectively), Karl filled on in bass for Canadian celtic-punk group The Real McKenzies.
Australian punk band Frenzal Rhomb wrote a song about Karl, on their 2011 album, Smoko At The Pet Food Factory, called 'Alvarez'.
Karl suffered a mild heart attack on August 11, 2007. He is now recovering and expected to tour with the Lemonheads later in 2007. A page has been set up to help cover medical expenses.