??Much like the bones in our bodies, the cytoskeleton of filamentous proteins largely determines the mechanical response and stability of cells. We describe advances in theoretical modeling and experiments on such fiber networks in living cells and on reconstituted in vitro acto-’?myosin networks. These networks exhibit rich zero-’?temperature critical behavior, characterized by diverging strain fluctuations and correlation lengths. Unlike passive materials, living cells are kept far out of equilibrium by metabolic processes and energy-’?consuming molecular motors that generate forces to drive the cellular machinery. We show how such internal force generation by motors can lead to dramatic mechanical effects, including a strong stiffening of cytoskeletal networks. Further, stochastic motor activity can give rise to diffusive-’?like motion in elastic networks, as observed in living cells.