  Cells and tissues exert forces and can actively change shape. This strikingly
autonomous behavior is powered by the cytoskeleton, which includes an active
gel of actin filaments, crosslinks, and myosin molecular motors. Although
individual motors are only a few nm in size and exert minute forces of a few
pN, cells spatially integrate the activity of an ensemble of motors to produce
larger contractile forces (order nN and greater) on cellular, tissue, and
organismal length scales (order 10 ${\mu}$m and greater). Experimental studies
of reconstituted active-network model systems have long suggested that a
mechanical interplay between the network's connectivity and motor activity
dominates contractile behavior. Recent theoretical models indicate that this
interplay can be understood in terms of percolation models, where the network
connectivity is influenced by motor activity. Here we review experimental and
theoretical studies of model contractile active gels and propose a state
diagram that unites a large body of experimental and theoretical observations
based on concepts from percolation theory.
