The timing belt is a notched rubber belt, sometimes called a Gilmer belt. This belt allows the crankshaft to drive the camshaft, which in turn opens and closes the valves. Without this belt, the engine can't run.

How do I know it's time to replace my timing belt?

A loose or worn belt will cause ticking or rattling noises, poor engine performance and overheating, usually triggering the check engine light. 

If the timing belt breaks, the engine can't run — and on some engines that break can cause internal damage. Most engines have timing chains, which typically don't require replacing.

How often should I replace my timing belt?

The schedule for replacing a timing belt varies by manufacturer, with some saying it should be every 60,000 miles and others 100,000 miles or more. 

Changing the timing belt requires removing many other parts, adding to labor costs. If the timing belt drives the water pump, many mechanics recommend replacing the pump at the same time.

Why do I need to change my timing belt?

It's not a question of why, but a question of how often, based on the recommendations of the vehicle manufacturer or a mechanic who finds the belt is stretched or damaged. 

See above for mileage recommendations, but remember: If it's broken, you're engine simply won't work.