Unable to determine simulator device to boot. - Xcode

I just downloaded Xcode 6 & played with it a bit.

After switching back to previous version of Xcode and running in simulator, I keep getting this error.

Unable to determine simulator device to boot.

How do we resolve this?

Thanks in advance.

Make sure you have only one iOS Simulator instance running in the Dock.

There was a simple answer to this for me:

1. Open the simulator via Xcode -> Open Developer Tool -> iOS simulator (even if it can't launch, the app will be open).
2. With the simulator app open, go to Hardware -> Device -> Manage Devices.
3. Add any missing simulators with the plus button (all of mine were missing for some reason).

I've faced same issue and solved by below steps:

1. Move xcode6-beta into application folder

2. Restart mac and open xcode6-beta from application folder.

I had the same problem when running from Xcode 6 beta selecting iPhone 5s. If I choose iPhone 5 or "resizable iPhone" then my app launches fine.

You will not see the profiles of simulator without a rebooting of OS X.I added simulator for many times,when the rebooting finished, it shows on the device list under the simulator bar. I think this is a bug of Xcode 6 Beta

I think you have 2 simulator running in the dock. Please close another one simulator and run the project again. It might helpful for you.

In my case I had such a message just because the simulated device was still booting (longer than usually, I guess). Xcode 7.2. (This is not actually an answer to this specific question, but I came here googling the message in the title, so I am writing my note here for if someone else gets the same case as I did)

My case wasn't about having two simulators open. It was just because having instruments open as well. Closing Instruments resolved the issue

I just fixed the exact same problem by removing the beta version and rebooting the computer.