Board Thread:Arrow discussions/@comment-9346948-20150811210531/@comment-9346948-20150815160836

Sean, they could crossover but it won't mean they're in the same universe. Just remember, with The Flash introducing the multiverse concept, that'll likely be how they will introduce Supergirl if at all. It wouldn't make sense for them to never ever heard of Superman or Supergirl or any of these other aliens.