We explore the effects of macroscopic shear stress applied to a filamentous polymer network. In particular, we aim to understand whether the network transforms affinely on microscopic length scales. Network deformations in response to external stress were measured by tracking fluorescent beads embedded in the networks, using the 3D imaging techniques. Networks of both flexible and semi-flexible polymers, for example, polyacrylamide and fibrin gels, are studied at strain values ranging from linear to nonlinear regime. (This work is supported by the MRSEC grant DMR-0520020, NSF grant DMR-0505048, and NSF MRSEC grant DMR00-79909.)