Testing your backup system is a critical, but often over-looked task. You need to monitor and test your backup system not only so you know that it’s properly copying your files, but more importantly so you learn how to use your backup system to retrieve a file. By practicing, you’ll be confident and ready when disaster strikes and you need to restore a file.

Time Machine is the easiest, most user-friendly backup program for restoring a file. Here are simple instructions, tested on Mac OS X 10.5 and 10.6, for restoring a file from Time Machine. We’ll imagine that you accidentally deleted a file named My Resume.doc from your Documents folder.

• Click on the Finder icon on your Dock. This will open a Finder window.
• Navigate to your Documents folder.
• Open Time Machine by clicking on the Time Machine icon in your menu bar and select “Enter Time Machine”
• Time Machine’s interface is a star field with a stack of windows disappearing into infinity. On the right-hand edge of the screen, you’ll see a timeline. Each of the hash marks corresponds to a different backup copy.
• On the right-hand time line, click on the date and time from which you want to restore your My Resume.doc file.
• You’ll see the top windows in the stack whisk by as Time Machine goes back to the date you selected. Notice that the date and time listed below the Documents window corresponds to the date and time you selected. You’re now looking at your Documents folder the way it looked at that point in the past!
• These windows behave like regular Finder windows. You can navigate around in them or use the search field. You can preview a file by double-clicking it. Click on your My Resume.doc file in your Documents folder to select it. (You can even select an entire folder or hold down the Command key and select multiple files or folders as well.)
• Click the Restore button in the lower right corner. If the name of the file you are restoring matches the name of a file currently stored in the same location, your Documents folder in this case, then Time Machine will ask you if you want to keep either the original item, the restored item or both.
• Your My Resume.doc file will be restored to your Documents folder.

Apple also provides an overview of Time Machine.

[Update October 2014: These steps still work on all OS X versions up to 10.9.]

[Update March 2018: These steps still work on all OS X versions up to 10.13.]