Ultrasonic thickness gaging is a widely used nondestructive technique for measuring the thickness of a material from one side. Just about anything made out of metal, plastic, composite, ceramic, glass, fiberglass, or rubber can be measured. This tutorial provides a basic introduction to the theory and practice of ultrasonic thickness gaging, for both newcomers and more experienced users who would like a review. It covers basic ultrasonic theory, how ultrasonic gages work and how they are used, and discusses a number of specific gage applications. Additional reference information including a glossary can be found in the Appendix.