William Frederick Rieflin (September 30, 1960 – March 24, 2020) was an American musician. Rieflin came to prominence in the 1990s mainly for his work as a drummer with groups (particularly in the industrial rock and industrial metal scenes) such as Ministry, the Revolting Cocks, Lard, KMFDM, Pigface, Swans, Chris Connelly, and Nine Inch Nails. He worked regularly with R.E.M. following the retirement of Bill Berry in 1997. He was a member of King Crimson from 2013 until his death in 2020.

Rieflin was born on September 30, 1960, and began his professional career in his hometown of Seattle. In 1975, he was in The Telepaths, a band which played backup for a couple of live gigs by The Tupperwares, which transitioned into The Screamers. He played drums for The Blackouts starting in 1979. His bandmates included Mike Davidson, Paul Barker, Roland Barker and Erich Werner. Eventually that band dissolved and Paul Barker joined the nascent Ministry.

Rieflin's earliest collaboration with Al Jourgensen was in 1986 on "You Often Forget", the second single by the Revolting Cocks. Later, he participated in the creation of Ministry's 1988 album The Land of Rape and Honey, and was noted for his performance in the live video In Case You Didn't Feel Like Showing Up (alongside fellow drummer Martin Atkins). Rieflin's work with Ministry and its side projects lasted through to the mid-nineties, though he noted that he was never credited as a member of Ministry proper, always as an "other" musician. Therefore, when he parted ways with the band during the Filth Pig sessions, he did not really quit since he was never an official member.

Rieflin helped Atkins kick off Pigface, the industrial collective that would grow to incorporate hundreds of artists, formed a friendship with labelmate Chris Connelly and founded First World Music. Like Connelly, Rieflin's work grew beyond his industrial roots. They collaborated on several recordings; two in particular, The Ultimate Seaside Companion (as "The Bells") and Largo, showcase Rieflin's keyboard skills.

Rieflin's solo debut, Birth of a Giant, featured him singing in something other than a background role, and also featured Robert Fripp. Improvisations from these sessions turned up later on the CD The Repercussions of Angelic Behavior, which was credited to Rieflin, Fripp and Trey Gunn.

Rieflin appeared on all KMFDM records released from 1995 to 2003 as a drummer, programmer, vocalist and keyboardist.^{[citation needed]} He toured with the band as a bassist in 2002 in support of the band's comeback album Attak and performed on the 2011 KMFDM album, WTF?!. He also drummed for Scott McCaughey's band, The Minus 5, which usually included R.E.M. guitarist Peter Buck.

Eventually Rieflin's work with The Minus 5 led to Buck offering him the opportunity to sit in with R.E.M., who had been missing a permanent drummer since the 1997 departure of Bill Berry. The band gave him the live drummer slot in its 2003 tour. They later announced that Rieflin would fill the role indefinitely, though once again as a hired musician rather than as an official member. In recordings, Rieflin also contributed bouzouki, keyboards and guitars to the group, serving as an auxiliary member until R.E.M. disbanded in 2011.

Rieflin, along with Scott McCaughey and Peter Buck, was a member of Robyn Hitchcock's backing band "The Venus 3", and appeared on three Hitchcock albums, from 2006 to 2010.