Jointed rock masses comprise interlocking angular particles or blocks of hard brittle material separated by discontinuity surfaces which may or may not be coated with weaker materials. The strength of such rock masses depends on the strength of the intact pieces and on their freedom of movement which, in turn, depends on the number, orientation, spacing and shear strength of the discontinuities. A complete understanding of this problem presents formidable theoretical and experimental problems and, hence, simplifying assumptions are required in order to provide a reasonable basis for estimating the strength of jointed rock masses for engineering design purposes. This paper summarizes some of the basic information upon which such simplifying assumptions can be made. A simple empirical failure criterion is presented and its application in engineering design is illustrated by means of a number of practical examples.