EPS (expanded polystyrene) foam is the standard liner material found in most ski helmets across all price points. It's proven technology that absorbs impact by permanently deforming—basically, the foam crushes to absorb energy so your head doesn't. It's lightweight, effective, and every EPS helmet still meets certified safety standards.
The most important thing to know: EPS is single-use. If you take a significant hit, the foam has done its job and won't protect you the same way again. You must replace the helmet after any serious impact, even if the damage isn't visible from the outside.
One downside is that EPS liners aren't removable or washable, unlike some premium options with merino wool covers. But don't let that deter you—EPS remains the most common and reliable choice for most skiers and riders. Focus on getting the right fit first, and consider MIPS rotational protection before worrying about liner upgrades.