The paper presents an engineering classification of rock masses for the design of tunnel support, based on an analysis of approximately 200 tunnel case records. The rock mass quality (Q) is a numerical value ranging from 0.001 (for exceptionally poor, squeezing ground) to 1000 (for exceptionally good, nearly unjointed rock). Q is determined by six parameters: RQD index, number of joint sets, roughness of the weakest joints, degree of alteration or filling along the weakest joints, rock load, and water inflow. These parameters collectively represent the rock block-size, interblock shear strength, and active stress. The proposed classification is illustrated with field examples and selected case records. Detailed analysis shows that suitable permanent support can be estimated for a wide range of rock qualities, based on Q, support pressure, and excavation dimensions and purpose. Support pressure is influenced by Q, joint roughness, and the number of joint sets, which determine the dilatency and freedom of the rock mass. Detailed recommendations for support measures include combinations of shotcrete, bolting, and cast concrete arches, with specified bolt spacings, lengths, and thicknesses. The boundary between self-supporting tunnels and those requiring permanent support can be determined from Q.The paper presents an engineering classification of rock masses for the design of tunnel support, based on an analysis of approximately 200 tunnel case records. The rock mass quality (Q) is a numerical value ranging from 0.001 (for exceptionally poor, squeezing ground) to 1000 (for exceptionally good, nearly unjointed rock). Q is determined by six parameters: RQD index, number of joint sets, roughness of the weakest joints, degree of alteration or filling along the weakest joints, rock load, and water inflow. These parameters collectively represent the rock block-size, interblock shear strength, and active stress. The proposed classification is illustrated with field examples and selected case records. Detailed analysis shows that suitable permanent support can be estimated for a wide range of rock qualities, based on Q, support pressure, and excavation dimensions and purpose. Support pressure is influenced by Q, joint roughness, and the number of joint sets, which determine the dilatency and freedom of the rock mass. Detailed recommendations for support measures include combinations of shotcrete, bolting, and cast concrete arches, with specified bolt spacings, lengths, and thicknesses. The boundary between self-supporting tunnels and those requiring permanent support can be determined from Q.