While the trick of using a custom lock screen image to display emergency contact info has been around since even before the iPhone existed, not everybody necessarily wants to clutter their lock screen with this information. The good news is that Apple now provides support for doing this in a slightly more usable way as part of the new Health app in iOS 8. Opening up the Health app will show a “Medical ID” button in the bottom right corner, in which you can fill in vital statistics such as name, photo, birthdate, medical conditions, notes, allergies and reactions, medications, blood type, weight, height, and several emergency contact names and phone numbers.
A “Show When Locked” option at the top of the screen allows you to ensure that this information is accessible when your iPhone is locked. Swiping to the passcode entry screen and tapping “Emergency” will show a “Medical ID” button in the bottom left corner below the keypad that can be used to bring up this information even when your iPhone is locked, and calls can also be placed directly to the emergency contacts by tapping on them, even when the iPhone is otherwise locked. Note that the “Medical ID” button isn’t the most discoverable, so if you have any serious medical conditions or are concerned about this information being accessible, it might still be a good idea to customize a wallpaper to at least tell users how to find the “Medical ID” section in the event of an emergency.