It's the first of four conferences at this, the E3 2015 day of conferences. Microsoft go first, as is tradition, and we can expect their show to be jam-packed full of Xbox talk. Don't worry, though, traditionally Microsoft also draft in plenty of third-party developers too, and so we can expect more than a couple of multi-platform announcements.
The whole thing kicks off at 5.30pm BST, or 9.30am PT. Basically, within the next hour. You can see the whole thing here, via the livestream embed below.
Update: It's over, but you can watch the archived stream on Twitch (opens in new tab).