The principle of neutron spin echo (NSE) spectroscopy is introduced. We focus our attention on the conventional Mezei type NSE spectroscopy. NSE achieves the highest energy resolution among other neutron scattering techniques, since energy resolutions of neutrons and energy transfer of a sample are decoupled with each other using neutron spin degrees of freedom as a measure of energy transfer of the sample. NSE naturally Fourier transforms the energy to time domain. NSE gives intermediate scattering functions, I(q,t), instead of dynamic structure factors, S(q,ω). Not only the quasi-elastic scattering but also elastic scattering can be encoded within the neutron spin.