Living cells are an active soft material with fascinating mechanical properties. Under mechanical loading, cells exhibit creep and stress relaxation behavior that follows a power-law response rather than a classical exponential response. Such a response puts cells in the context of soft colloidal glasses and other disordered metastable materials that share the same properties. In cells, however, both the power-law exponent and stiffness are related to the contractile prestress in the cytoskeleton. In addition, cells are made of a highly nonlinear material that stiffens and fluidizes under mechanical stress. They show active and adaptive mechanical behavior such as contraction and remodeling that sets them apart from any other nonliving material. Strikingly, all these observations can be linked by simple relationships with the power-law exponent as the only organizing parameter. Current theoretical models capture specific facets of cell mechanical behavior, but a comprehensive understanding is still emerging.