Intelligent System (IS) technologies have shown encouraging potential in facilitating very fast power system dynamic security assessment (DSA) in recent years. In general, the development of the IS consists of four stages. This paper conducts a comprehensive and detailed review on each stage and classifies the methods involved in the stages. The classification highlights the differences between the methods and show their advantages and deficiencies. The value of this paper is that it can serve as a useful and practical guideline to assist in developing ISs for very fast DSA. Moreover, the proposed classifications can provide deep insight into the enhancement of the IS towards improved DSA performance.