Glee star Grant Gustin has been cast as The Flash in the upcoming series that the CW is developing. The Flash, a.k.a. Barry Allen, will be introduced in Arrow before the actual series takes off. He will appear in 3 episodes of Arrow. According to THR, it will be episodes eight, nine, and twenty, "the latter of which will serve as the backdoor pilot for a potential spinoff Flash series."
The spin-off series is being developed by Arrow co-creators Greg Berlanti and Andrew Kreisberg, as well as director David Nutter. Berlanti, Kreisberg, and DC Entertainment's Geoff Johns will write the script for the series.
These guys have all done a great job with Arrow, and I hope they can find a way to make Flash cool, because I personally think Flash is one of the lamest superheroes out there.