It is the same rubber material as the rest of the shoe. It is similar to the insoles of Crocs, only I find these more comfortable. I would live in them except that they become slippery inside when wet and I am afraid I will slip and fall. Other than that they are fabulous. I have plantar fasciitis in my right foot and fat pad atrophy in my left foot and have had to wear orthotic inserts in every shoe for the past 10 years, but not with these shoes. They are truly comfortable. The only other con that I can think of is that they may get uncomfortable in the summer if your feet perspire.