Rashguards serve their primary role as a barrier between skin to skin or skin to mat contact when you're training. Originating in the surf community, we now see rash vests as very popular items in MMA and BJJ gyms around the world. The build from nylon, lycra or spandex allows them to be tight-fitting and flexible, therefore helping to eliminate injuries with extremities getting caught up - as you'd see with cotton shirts and the like - as well as allowing sweat to wick out. A good quality MMA rashguard will stand the test of time whilst helping to stave off some of the infections that crop up in gyms occasionally, and it will also help prevent mat burn. Mostly worn for training either on their own or under a gi, many BJJ and submission wrestling competitions now make use of coloured rash guards to denote belt level in their competitions.