The genesis of the group started in August 1997 when HBK cost Undertaker the WWF World championship. Shortly thereafter, Shawn had a TV match with Mankind (who was feuding with HHH) in which HHH and Chyna came down to ringside and Rick Rude wound up helping HBK score the win. The following week, HBK and HHH wrestled Undertaker and Mankind in a tag match, which made their union official. The name of the group was inspired by Bret Hart, who had been referring to Shawn as a "degenerate" as far back as February 1997.
After WrestleMania XIV, Shawn Michaels left the WWF, and HHH created The DX Army (which was interchangable with the name Degeneration X) with Chyna, X-Pac, and the New Age Outlaws. As the group's antics became more crowd-pleasing and less disruptive, they became fan favorites. The group remained united until the night after the Royal Rumble in January 1999 when Chyna double-crossed HHH. Her defection ended at WrestleMania XV, or at least it seemed until HHH and Chyna cost X-Pac his match. HHH went on to win the WWF championship in August 1999.
Degeneration X reformed as heels in October 1999, but their reunion was less impactful, and the group completely scaled back to Road Dogg and X-Pac through August 2000 before the two went separate ways. The past members of DX continued to interact, but an official reunion was not seen until June 2006 when Shawn Michaels and HHH reunited as mutual adversaries of Vince McMahon. Since then, DX has been used off-and-on, usually whenever both men are on the same brand and neither man is on the DL.