When an iPhone application developer wants to send you pre-release software to test, the software must include a certificate that allows it to run on your particular device. This is how Apple assures that only approved software can run on the iPhone or iPod Touch. Your device is identified by a cleverly-named long string of numbers and letters called a device identifier.

This is a (fake) example of a device identifier: 7c8f223423cb1be7995591d2a21232a80c331313

Here is how to find your device identifier.

1. Launch iTunes and connect your iPhone or iPod Touch.  (Mac or PC.)

2. Click on the “Serial Number” label on the iTunes window.  (See picture below.)

3. “Serial Number” will change to “Identifier (UDID)” followed by the device identifier.

4. Choose the Edit menu, then click Copy (or use Command-C  on the Mac or Control-C on Windows).  The device identifier is now on your clipboard.  (There is no need to select anything beforehand.)

5. Paste it into an email and send it to your favorite developer.

You’re done!

Click on "Serial Number". It will change to "Identifier (UDID)". Then copy and you're done!