AbstractRock mass classification systems (RMCS) were developed in the seventies of the last century. They were born in response to the need to establish indicative design criteria for tunnel support. But they also focused on rating the quality of the rock, putting numbers in geology that allowed the rock mass to be classified and its parameters computed. RMCS attempted to consider the most relevant geomechanical aspects affecting the rock mass, including parameters related to rock structure and joint surface conditions. However, to be used as design approaches, they covered parameters not related to the rock mass itself (excavation orientation, stress state…). This multiple focus, together with other intrinsic limitations, has introduced some confusion in its application. A discussion is presented on the application of RMCS to solve rock engineering problems based on the author’s experience, addressing the geological background and their applications. The author reports on some small-scale rock engineering studies including the results of basic RMCS, presenting the potential problems encountered, if any, and the usefulness of the approach in relation to the objectives of the study. While a “design as you go” approach, based on RMCS of the tunnel face combined with observations, measurements and expertise, has often been successful, its direct application for design purposes without further considerations tends to produce problems. Very useful and almost necessary for comparative purposes, these simple approaches may, however, not always produce the right answers to design questions or may lead to wrong decisions if used without additional thoughtful considerations.