Mechanical properties of bio-polymer gels are important in understanding how cells move in surrounding tissues. Strong evidences from experiments suggest that the locomotion of cells is very sensitive to the alignment of filaments in the extracellular matrix (ECM), and cells also act on the ECM to remodel it. In typical biological processes the deformations of the ECM enter deeply into the nonlinear regime, and thus a systematic understanding of the nonlinear elasticity of the ECM is called for in order to understand cell motility. In this talk I will discuss a Landau theory we proposed recently, which captures the filament alignment with a nematic-like order parameter and explains the nonlinear elasticity of bio-polymer gels. I will show that a number of important effects of the ECM follow naturally from this approach, proving its potential for modeling cell motility.