In sequence-controlled polymers, repeat units of different chemical composition are arranged in a well-defined order. Due to its living characteristics and functional group tolerance, ruthenium-mediated ring-opening metathesis polymerization (ROMP) is a powerful strategy for sequence control. Catalysts for ROMP can be tuned to achieve polymers with highly controlled sequences, thereby encoding structural information in the structure of the catalyst itself. The study of sequence-cotrolled polymerization offers both mechanistic insights and routes to new materials with precise structure-property relationships. This chapter will review design strategies for ruthenium-mediated sequence-controlled ROMP. Structural control in terms of cis/trans-selectivity and tacticity will first be discussed to illustrate the key principles of catalyst design. This framework will then be extended to sequence control for alternating copolymerization, with the goal of motivating continued progress in this area.