In this talk, we establish global existence of solutions of the liquid crystal (gradient) flow for the well-known Oseen-Frank model. The liquid crystal flow is a prototype of equations from the Ericksen-Leslie system in the hydrodynamic theory and generalizes the heat flow for harmonic maps into the 2-sphere. The Ericksen-Leslie system is a system of the Navier-Stokes equations coupled with the liquid crystal flow. In the second part, we also prove global existence of solutions of the Ericksen-Leslie system for a general Oseen-Frank model in 2-D. (This is my joint work with Prof. Zhou-Ping Xin.)