Buffy creator Joss Whedon is a huge comics fan, particularly the Marvel comic X-Men. His dream came true when, post-Buffy, Joss wrote the Astonishing X-Men comics. The comic has been referenced numerous times in Buffy:

• In No Place Like Home, Riley says, “Giles, you got that Danger Room set up out back?” The Danger Room was a training room in the X-Men.
• In Tough Love, Xander reads an X-Men comic while in the Magic Box.
• In Two to Go, Jonathan says Willow is like the “Dark Phoenix” who was a character in the X-Men comics.
• In Him, Xander calls Spike a “nimrod”. Xander himself was called this by a soldier in Innocence, and Spike called Warren a nimrod in Smashed. Nimrod was a figure in ancient Babylon, and was also an X-Men character.
• In Bring on the Night, Andrew mentions Apocalypse, who was also an X-Men character (”Okay, I know what you’re thinking. Andrew, bad guy. You think I’m a super-villain like Dr. Doom or Apocalypse or The Riddler.”)
• Gwen Raiden in Angel’s season four appears to have been inspired by Rogue from the X Men - the two had great powers which could harm other humans by touch, and had to wear gloves to protect others.
• Joss Whedon has said that the X-Men character Kitty Pryde (AKA Shadowcat) was a large influence for the character of Buffy.