Living cells move, deform and divide. The engine of these behaviors is the cytoskeleton, a highly crosslinked network of polymer filaments and molecular scale motors that use chemical energy to do work. We develop theory that predicts how the micro-scale properties of molecular motors and crosslinks tune the networks emergent material properties and generate predictable, and possibly controllable, behaviors. I will present how this theory is constructed, and show detailed comparison with a numerical implementation of the same system. I will then discuss its implications for cytoskeletal networks in vitro and in vivo. In particular I will demonstrate how our theory can usefully describe important aspect of mitotic spindles and contractile actomyosin cortices.