The two main materials used for IWB holsters are Kydex and leather. Kydex is a durable, lightweight thermoplastic that doesn't require a break-in period and retains its shape well, making re-holstering easier. However, it can be less comfortable against the skin and may cause wear on the firearm finish over time. Leather holsters offer a comfortable, classic fit but require a break-in period and may retain moisture. They also might not maintain their shape as well over time, making re-holstering more difficult.