In this talk, we will talk about the dynamics (mainly scattering theory) for critical nonlinear Schrodinger equations. First, we will introduce the background, some important results and some famous conjectures in this area. Moreover, two specific results of the speaker will be addressed, i.e. ``Scattering for the high dimensional inter-critical NLS'' (joint with C. Gao) and ``Dynamics of subcritical threshold solutions for energy-critical NLS'' (joint with Q. Su).