Every fingerprint is unique, so it’s rare that even a small section of two separate fingerprints are alike enough to register as a match for Touch ID. The probability of this happening is 1 in 50,000 with a single, enrolled finger. And Touch ID allows only five unsuccessful fingerprint match attempts before you must enter your password. By comparison, the odds of guessing a typical 4-digit passcode are 1 in 10,000. Although some codes, like “1234,” might be more easily guessed, there is no such thing as an easily guessable fingerprint pattern.

To start using Touch ID, you must first set up a passcode on your iPhone or iPad (or a password on your Mac). You must enter your passcode or password for additional security validation:

after you restart your iPhone, iPad, or Mac;

when more than 48 hours have passed from the last time you unlocked your device;

to add or delete a fingerprint to use with Touch ID;

to change the iPhone or iPad passcode or Mac system password, and for other security settings like FileVault on your Mac;

when there have been more than five unrecognized Touch ID authorization attempts in a row; and

after you log out of your Mac.

To improve security, you can choose a long, complex alphanumeric password. On your iPhone or iPad, tap ‘Passcode Options’ and select Custom Alphanumeric Code.

If your device is lost or stolen, you can prevent Touch ID from being used to unlock your device with Find My iPhone Lost Mode. Starting with iOS 7, your iPhone and iPad offer additional protection against theft with Activation Lock, which requires an Apple ID and password to turn off Find My iPhone, erase data, or reactivate your device. If your MacBook Pro with Touch ID is lost or stolen, erasing your Mac remotely also disables Touch ID.

You can also use Touch ID to purchase content from the iTunes Store, App Store, and iBooks Store, instead of entering your Apple ID password.

Touch ID can be used by multiple users on a MacBook Pro, making it easy to share a system securely. Each user account can have up to three enrolled fingerprints, and a total of five fingerprints can be enrolled across the system.