Abstract: With protein being one of the most important components in biological systems, it is essential to know the dynamics of the internal motions of proteins for further understanding of the mechanisms of life. Utilizing the unique properties of thermal neutron scattering with hydrogen isotopes to study the dynamics inside proteins takes full advantage of the low beam energy and high spatiotemporal resolution of neutron spin echo spectroscopy over other experimental methods. This cutting-edge discipline spans nuclear physics and technology, non-equilibrium statistical physics, molecular biology, and proteomics. There are plenty of unknowns and challenges, as well as opportunities to peek at the mysteries of life. In this article, we review the principles of neutron spin echo spectroscopy, the history of its development and its advantages with some examples of experiments that have been performed on protein domain dynamics, then discuss various future applications.