Liner:

Most helmet liners are made of expanded polystyrene foam. On impact, the liner dissipates the force of the impact to protect your head. Make sure the liner fits your head comfortably and that it's not damaged or dented.

Shell:

Most cycling helmets are covered with a plastic shell to hold the helmet together in a crash, provide puncture-resistance and allow the helmet to slide on impact (to protect your head and neck). Make sure the shell is intact and in good shape.

Ventilation:

Helmet vents enhance wind-flow over your head, keeping you cooler and more comfortable as you ride. In general, the more vents you have, the cooler you'll be.

Straps:

Find a helmet strap system that's comfortable and easy to get in and out of. Also make sure that it matches your riding style — beefy for rough terrain and mountain trails, lighter and cooler for road racing. Try a couple of different systems before you decide on one.

Hair port:

This helmets come with a strap design that accommodates ponytails

Adjust the Fit

Whatever your size, simple adjustments can customize the fit. Nearly all helmets offer an internal universal-fit sizing ring (one size fits all) and adjustable straps. Occasionally some helmets, including some kids' helmets, still offer a selection of fit pads to accomplish this task.

To adjust the fit, first expand the sizing ring before you place a helmet on your head. Once the helmet is in place, reach behind your head and tighten the ring (by pushing a slider or twisting a dial) until you achieve a snug fit.

Right: Helmet is level 
 A good-fitting helmet should be snug but not annoyingly tight. It should sit level on your head (not tilted back) with the front edge no more than 1 inch above your eyebrows. Your forehead should be protected. Push the helmet from side to side and back to front. If the helmet shifts noticeably, adjust the sizing ring (or pads) to snug the fit.

Wrong: Helmet is tilted back 

Next, buckle and tighten the chinstrap. Push up on the front edge of the helmet, then up on the back edge. If the helmet moves significantly in either direction, tighten the chinstrap and try again. Adjust the straps around both ears to achieve a comfortable fit. Finally, with the chinstrap buckled, open your mouth. If the helmet doesn't press against your forehead as you do so, tighten further and repeat. Just don't tighten the strap excessively and create discomfort.