The advance of mechanics towards the frontiers of biology and soft matter necessitates an understanding of entropic forces which manifest themselves as thermal fluctuations at length scales of a micron and below. The effects of these fluctuations can be easily seen in biofilaments at thermodynamic equilibrium under the action of forces and moments as they fluctuate around their minimum energy configuration due to Brownian motion. This remains true of filaments in networks and gels such as those of actin, spectrin, fibrin or other biopolymers. The thermal motion of these filaments at the microscopic scales manifests itself as entropic elasticity at the macroscopic scales.