Lithography is the most important equipment in the manufacturing of very large-scale integrated circuits. Resolution is one of the important performance metrics of lithography. As an important means to improve the resolution of lithography, resolution enhancement techniques can promote the development of chips' integration. As one of resolution enhancement techniques, source and mask optimization (SMO) is indispensable in 28 nm technology node and beyond. It improves the resolution by simultaneously optimizing the source and mask. Accurate representation of source and mask is the basis of SMO, and efficient optimization algorithm is the core of SMO. The premise of full chip SMO is critical pattern selection. In this paper, the development history of SMO is reviewed. Combined with the research work of our group, the basic principles and research progress of the critical pattern selection method, source mask representation method, and optimization algorithm are reviewed.