Studies by Bausch and colleagues have led to the development of a simplified model system, which consists of filamentous actin, crosslinking proteins and molecular motors. The result of motor activity in a system of crosslinked actin filaments is a rich, dynamical regime in which clusters of filaments constantly form, grow, and dissolve. Bausch and co-authors show that a steady-state regime requires a finely tuned system at the edge of stability, between fluidization and permanent network stability. They also show results from a simple computer-simulation model that seems to be able to reproduce the observed structural patterns and dynamics, including the need for marginal stability of the networks. Similarly, Martin et al. observed myosin cluster formation and coalescence in Drosophila embryos that appeared to depend on actin filaments. Such cellular processes may be examples of the kind of nucleation and growth proposed by Bausch and co-authors.