Crosslinked and bundled actin filaments form networks that are essential<br> for the mechanical properties of living cells. Reconstituted actin<br> networks have been extensively studied not only as a model system for<br> the cytoskeleton, but also to understand the interplay between<br> microscopic structure and macroscopic viscoelastic properties of<br> network-forming soft materials. These constitute a broad class of<br> materials with countless applications in science and industry. So far,<br> it has been widely assumed that reconstituted actin networks represent<br> equilibrium structures. Here, we show that fully polymerized<br> actin/fascin bundle networks exhibit surprising age-dependent changes in<br> their viscoelastic properties and spontaneous dynamics, a feature<br> strongly reminiscent of out-of-equilibrium, or glassy, soft materials.<br> Using a combination of rheology, confocal microscopy and space-resolved<br> dynamic light scattering, we demonstrate that actin networks build up<br> stress during their formation and then slowly relax towards equilibrium<br> owing to the unbinding dynamics of the crosslinking molecules.