If you pre-ordered Overwatch [official site] before April 29th, it's possible you're already playing its latest multiplayer shooty beta. If you didn't and you still want to play, you'll instead be hanging on until the open beta begins on May 5th. In any case, the beta then runs until May 9th. Read on for more exact start and stop times.
When does the Overwatch early access beta start?
If you pre-ordered the Overwatch: Origins Edition of the game, then you'll have entry to the 'early access' beta. That began May 2nd at midnight BST / 4pm PDT / 11pm UTC, which means you can go play right now.
When does the Overwatch open beta start?
For everyone else, you'll be able to access the game on May 4th at midnight BST / 4pm PDT / 11pm UTC.
When does the Overwatch open beta end?
And your access will end on May 9th at 6pm BST / 10am PDT / 5pm UTC.
In any case, as with all Blizzard games you'll need a Battle.net account and the client in order to install and play the game. Overwatch should be present on your games list after the beta starts, ready to be downloaded.
This is the last beta before Overwatch is released on May 24th. This feels like it's been a long time coming, but I'm not sure why. It wasn't that long ago that the first beta began. Here's what Pip said at the time, with the caveat that the game will change, and has changed, a lot since. We'll have more on Overwatch in the coming weeks, natch.