During the last twenty years, the theory and application of built-in test (BIT) have made prominent progress. The BIT technology has been the effective method of improving product's testability and diagnosis ability. The paper summarizes the trait of BIT technology, analyzes its developing trends all over the world, and generalizes the intelligence of BIT during its phrases of system design, information process and synthetic decision, then synthetically analyzes the application to BIT of expert system, neural network, fuzzy theoetic, information fusion and other intelligent theories and builds up the theory frame of intelligent BIT.