Kazhdan-Lusztig theory is one of the most important developments in representation theory of algebraic groups during the last 40 years. The theory plays an essential role in classification of irreducible characters of finite reductive groups and in determining characters of certain irreducible representations of highest weight. Meanwhile, the theory opens many research topics such as Kazhdan-Lusztig polynomials, Kazhdan-Lusztig cells of Coxeter groups, asympototi Hecke algebras, relations between Coxeter groups and K-theory and perverse sheaves. In this talk we will give a brief introduction to its origin, development, impact and some open problems.