In preparation for the incoming Fallout 4, Bethesda will be releasing an iOS/Android app titled Fallout Shelter. Fallout Shelter will play much like other simulation games, with resource and people management playing a big role in the game with players managing their own Vault.
The game is inspired by games like X-Com, which was apparent in the Vault layout looking fairly similar to the base in X-Com.
Fallout Shelter will not require an internet connection to be played. As an added surprise, it was announced that Fallout Shelter will be releasing tonight (June 14th) as soon as the E3 showcase is over.
Edit: App is now live! Head to the Apple App Store or the Google Play Store to download.