This paper described the functions, developing course, classification, future trends, characteristics, and the construction technologies of crop expert system. The problems and challenges of developing crop expert system were also discussed. There is a need to combine and integrate the knowledge from different specialties for the development of crop expert system. Knowledge database is the key of establishing crop expert system, and the knowledge should be modeled. The foundation for building knowledge model of crop expert system is the crop growth-development time and production management technique. The crop expert system should be able to fulfill the requirements of different types of users: researchers, extensionists, and farmers. The developing process and long-term experience in Taiwan Rice Information System could be provided as a reference for the development of knowledge database in other crop expert systems.