Hence why it has been such an exciting time to watch the DC / CW shows ‘Arrow,’ ‘The Flash,’ ‘Legends of Tomorrow,’ and now ‘Supergirl,’ as they are all part of the same universe (though ‘Supergirl’ at the moment is just part of the same multi-verse) and we know everyone involved in these shows is working hard to tie them all together and create interesting cross-over experiences.
Source: Super Series: This Year’s CW/DC Cross-Over Event Promises To Be A “3 Hour Story”