A bike helmet doesn't last forever. That means if you want to protect your head as well as you can (and you should -- head injuries can be life-ending), you need to replace your helmet every few years.
First and foremost, if you've ever been in a crash and the bike helmet has been involved in an impact, replace it -- even if you can't see any visible damage. Its structural integrity has probably been compromised.
Most manufacturers will tell you to replace your helmet every 3 to 5 years. The label inside the helmet will tell give you the date when it was manufactured. Some even give you a "best by" date. Sunlight, temperature changes and moisture will take a toll on the helmet's materials, so it's smart to update every few seasons, even if it hasn't been involved in a crash.
The label or a separate sticker will tell you if it's certified to meet CPSC, ASTM or Snell standards. Those are the most current safety regulations, so if it's not meeting one of those, it's time to for a change.
If all else fails, use this test: Can you remember when you bought your current helmet? If not, you probably need to switch!
To help your helmet last a little longer, read our easy Helmet cleaning tutorial!