Computerized adaptive testing (CAT) has unsurpassable advantages over traditional testing. It has become the mainstream in large scale examinations in modern society. This paper gives a brief introduction to CAT including differences between traditional testing and CAT, the principles of CAT, psychometric theory and computer algorithms of CAT, the advantages and cautions of CAT. In the end, the development of CAT in China is reviewed. (Contains 1 figure and 1 table.)