http://www.tvinsider.com/article/70539/the-flash-grant-gustin-to-guest-star-on-supergirl/

Grant Gustin's Barry Allen will drop by Melissa Benoist’s superhero series on March 28. The crossover (http://www.tvinsider.com/article/82408/supergirl-the-flash-crossover-came-together/) is "something that we have dreamed of happening since we starting making Supergirl," said Greg Berlanti and Andrew Kreisberg, who are executive producers on both series.

http://i1331.photobucket.com/albums/w594/JamesGrec3/Supergirl_zpshlplqqe1.png

http://www.comicbookresources.com/article/flash-heading-supergirl-but-how-will-supergirl-tie-into-flash

On March 28, worlds will collide when The CW's Flash -- Grant Gustin -- shows up from an alternate universe to team with CBS' Supergirl, Melissa Benoist. This isn't the first time the shows have crossed over, either; on the February 9 episode of "The Flash," Barry Allen glimpsed Supergirl as he ran through a breach between his universe (Earth-1) and the parallel universe of Earth-2. So we know Barry will probably run through another breach, and somehow end up on "Supergirl." But the big question no one seems to be asking is, how will this tie back in to "The Flash?" If, that is, it does at all? Viewers will find out how it all works soon enough, but in the meantime, we can speculate wildly on

http://www.cinemablend.com/television/Why-Didn-t-Flash-Talk-About-Supergirl-Crossover-All-132957.html

Warning: spoilers for this week’s episode of The Flash are ahead! After a three-week hiatus, The Flash returned to our screens this past Tuesday with its 18th episode, titled “Versus Zoom.” Although we learned that Jay Garrick, a.k.a. Hunter Zolomon, was Zoom in “King Shark,” this episode gave background on the Season 2 main baddie’s past and his motivations. Admittedly, the episode left a lot more questions than answers, but one of the bigger questions didn’t have anything to Zoom. What we found ourselves confounded by was why there was no mention of last month’s Supergirl crossover whatsoever. Let’s wind the clock back to March 28, when Barry Allen helped Kara Zor-El battle Silver Banshee and ...

Now lets' see Super Girl crossing over to The Flash.