Swim trunks should be made out of a material with stretch that is suitable for being in the water.

Typically, this falls apart in a few categories:

- Nylon mixed with elastene, spandex, or lycra is soft and stretchy. This is what most casual swimwear is made from.
- Polyester mixed with PBT (polybutylene terephthalate) is less soft to the touch, but resistant to chlorine and salt water. This is what a lot of competitive swimwear is made from.
- Neoprene, also known as scuba, is heavier and less tretchy. It's the stuff scuba suits are made from.