After plenty of fan speculation and eternal optimism, Grant Gustin's Barry Allen has finally been confirmed to appear in an upcoming episode of CBS' Supergirl. Just the prospect of seeing The Flash sharing the small screen with the Girl of Steel is enough to get fanboys grinning from ear to ear, but there's so much potential in terms of what we can and (hopefully) will see play out between these two heroes. The fact that Gustin and Melissa Benoist know each other from their Glee days means that chemistry shouldn't be a problem for the two, and the energetic, fun tones of The Flash and Supergirl as shows should make for a great experience when the Scarlet Speedster arrives in National City. We don't know for certain how The Flash's appearance in Supergirl will play out, as in whether this 100% definitely means that Kara exists in a world that features The CW's Flash, Green Arrow, Atom, Firestorm, etc or if it could be a case of Barry possibly using his speed to hop to an entirely different world - a world that features CBS' Supergirl. Regardless, there's a whole heap of optimism for what lies ahead for Barry Allen when he crashes Supergirl's party next month. Based on what we know of both shows, their shared creators, and the acting talent involved, there are some things we should definitely expect from the Sultan of Speed's arrival in the world of Kara Zor-El.