The free download is designed to replicate the in-game Pip-Boy on iOS and Android devices, and interacts with your PC, PlayStation 4 or Xbox One if you've got Fallout 4.
Right now, five days away from the launch of Fallout 4, the app doesn't do much. It works in "demo" mode, meaning you can explore its innards, such as your inventory, and play Atomic Command, Fallout's version of retro arcade game Missile Command.
You can scroll around Fallout 4's map, although, as you'd expect, nothing is marked at this stage except Vault 111.
You can resize the screen by setting the display to "hardware". This is for those who wish to slot their smartphones into the Pip-Boy accessory that comes with the Fallout 4 Pip-Boy Edition.
When the app links up to Fallout 4, it'll remember your status and let you access your data on the go.