Most biological materials, for example the cell cytoskeleton and extracellular matrix, are hydrogels composed of semiflexible biopolymers such as F-actin, microtubules and collagen. The biological materials distinguish themselves from synthetic soft materials by their nonlinear viscoelasticity, usually characterized by a large increase in elastic modulus at increasing strains. The nonlinear elasticity is essential for normal function of cells in soft tissues. However, the mechanism that leads to this nonlinear elasticity in gels of semiflexible polymers remains unclear. In this talk I will present recent measurements on nonaffine deformations and filament alignments in polymer networks and discuss the possible mechanisms for nonlinear elasticity and negative normal stress in biopolymer gels. I will also show in vivo measurements of cellular elasticity as a function of the stiffness of the culture substrates and discuss how cells respond to the mechanical properties of their environment. Qi Wen is a post-doc at the University of Pennsylvania. He obtained his Ph.D. degree in Biological Physics from Brown University in 2007