100% cotton.  Very thin cotton.  Don't expect anything thick, heavy, and durable.  For me, it's the perfect light weight t-shirt to wear on a hot day.  I wear mine under my leather motorcycle jacket like Fonzie did.  Why do I buy these?  Because I don't want to wear t-shirts emblazoned with someone's advertising.  Not all of us need to wear logos in order to impress upon others how much we spend on our clothes.  But the most important part is 100% cotton.  That means no polyester.