In this talk, I will discuss a simplified Ericksen-Leslie system modeling
the hydrodynamics of nematic liquid crystals, that is coupling between Navier-Stokes equations and harmonic map heat flows. I will describe some existence results of global weak solutions in dimensions two and three, and a finite time singularity result in dimension three. This is based on some joint works with Tao Huang, Junyu Lin, Fanghua Lin, and Chun Lin.