A wealth of physical phenomena emerges from energy-consuming constituents obeying simple interaction rules. When these active particles are free to move, they form active fluids, an exciting area of research which has already impacted fields from human crowd engineering to the nanoscience of molecular motors. Recently, significant scientific advances have focused on active matter where the constituents form an elastic solid mesh. Examples of active solids include biological tissues, the cytoskeletal polymer network that gives living cells their mechanical properties, quantum solids driven far from equilibrium using light, and artificially engineered structures in which collections of robots (from the nanoscale to the macroscale) are coupled together to form a lattice. A common theme across all of these systems is the harnessing of far-from-equilibrium physics to design materials that can perform specific functions, as well as the ability of these active systems to dynamically reconfigure themselves to achieve multi-functionality.