1.5mm, constructed of 100% FluidFlex, ONeill's New and Exclusive High-Stretch Neoprene. The Hammer Series Now Provides Completely Unrestricted Movement and a Perfect Fit. This vest design has proven itself through the years. It has no zippers, just pull on and off. You know it is summer when this vest is on and your paddling out at dog patch on your favorite Harbour San-o model!