From the Stone Age to the Silicon Age, the materials available to humankind have defined the world we live in. Tomorrow, our world will be shaped by the design and integration of a host of building blocks into materials systems dictated by the conception of new technologies. In the coming Age, matter will be realized on demand – what, where and when we need it -- by engineering the assembly of multifunctional collections of building blocks into materials with precision, programmability and personalization. One important route to matter-on-demand is through colloidal nanoparticles designed and synthesized as multi-material "atoms" with the valency needed to assemble into complex target structures. Unlike electronic valency governing the assembly of atoms through chemical bonding, nanoparticle valency arises from anisotropy in particle shape and physical interparticle interactions – both of which can be designed and engineered. This valency is tunable and leads to structures of extraordinary diversity, complexity and utility. By making particles active, smart, and morphable, the opportunities for complex particle systems to achieve functions inspired by biology start to become possible.  In this lecture, I discuss the patchy particle and “digital alchemy” frameworks we use to predict and design complex particle systems, and explore how complex particle systems may evolve in the future.