The materials we use today for mechanical design are the outcome of at least 3000 years of development, much of it empirical but much the outcome of systematic science. Both approaches have been motivated by the desire for stiffer, stronger, more durable, and lighter structures, progressively populating material property "space". We first examine the extent to which this space is now filled and estimate the ultimate constraints on this filling. Strategies for expanding the filled regions further include hybrid material design-making materials by combining two or more monolithic materials in chosen configurations and connectivity. Here we explore two classes of hybrid-lattice materials and sandwich structures-developing ways of comparing their properties with those of conventional monolithic materials. The comparison reveals the potential of hybrids. This paper builds on ideas that have grown from a long and rewarding collaboration with Tony Evans with whom it has been a privilege to have worked.