The spectral theory of quasiperiodic operators is a fascinating field which continuously attracts a lot of attentions for its rich background in quantum physics as well as its rich connections with many mathematical theories and methods. In this lecture, I will briefly introduce the main problems in this field and their connections with dynamical systems, especially the reducibility theory. Finally, I will talk about a recent result by Avila-You-Zhou on the phase transition in Almost Mathieu Operators which is an application of dynamical systems method in the spectral theory.