Your noggin is the #1 thing to protect when playing derby. To minimize the chances of concussions, a good helmet is a must. There are a few different things to look for when it comes to helmets for roller derby. 

First (and most importantly), helmets for derby should be dual certified for multiple impacts. There will be stickers on the helmet indicating that it meets the U.S. CPSC Safety Standard and the ASTM F-1492 Skate safety standards.  Helmets without these certifications will not have adequate protection for the skating, bumping, and falling that happens in roller derby. 

Secondly, helmets need to be in good shape. This means that the foam inside is in tact, and that there are no cracks anywhere on the helmet. 

Finally, helmets have an expiration date. It's best to replace your helmet every 3-5 if they don't recieve any impacts. However, if you knock your head even just once while wearing your helmet, it's time to replace it.