This paper presents a state-of-the-art review of the Geomechanics Classification of rock masses, also known as the RMR System. Developed by the author in 1972–1973, and modified and improved as more case histories became available, the latest version of the system was published in 1979. This paper discusses the principles and philosophy of the RMR System and describes its varied applications. Although originally directed to tunneling in rock, subsequent applications and extensions featured the use of the RMR System in hard rock mining, coal mining, rock slope stability, foundations in rock, rock boreability, and others. These applications necessitated some adjustments to the classification ratings, and these aspects are presented in the paper. The strengths and limitations of the Geomechanics Classification are pointed out. It is emphasized that rock classifications are useful aids in engineering design, but that they form only a part of the design process.