Electron micrographs of the cortical cytoskeleton reveal a dense meshwork of actin filaments, bound together by regulatory proteins (a). Professor Geissler and postdoc Sander Pronk have devised an efficient computational model for such a biopolymer gel, in essence a disordered network of stiff elastic filaments linked at discrete nodes. One realization of an idealized network is depicted in (b). The thickness of lines connecting nodes is proportional to the instantaneous strain in this configuration typifying thermal equilibrium. This result, illustrating the heterogeneous response to elastic deformations, was obtained by sampling an ensemble of thermal fluctuations via the Metropolis Monte Carlo algorithm. Trial moves in these simulations included translation and rotation of individual nodes (c), as well as compressive and shear strain of randomly selected subsystems. Physiological networks like that pictured in (d) are often far from equilibrium and, when growing in a particular direction (e), can exert substantial net forces. In order to capture the collective motions underlying force generation and stress response, the trial move set must be augmented by attachment of nucleation promoters and polymerization at filaments' free ends (f). Professor Dan Fletcher's laboratory performs detailed experiments on the mechanics of growing actin networks, which complement, inspire, and test these calculations.