Abstract: Long distance pipelines, which are mostly buried underground, can only be checked by means ot internal inspection, therefore, pipeline internal inspection is of great importance. In foreign countries, pipelines are internally inspected by pigs, which lefer to any device driven with the fluid transported to run through pipeline. Pigs can be classified into two categories: the pigs used in construction phase and those used in operation pha.se. Introduced are the pigs used for different purposes, with emphasis stressed on the introduction to the development and application of geometric detectors in foreign countries. Magnetic leakage pigs and ultrasonic pigs used for detecting pipe wall thickness are compared and analysed in terms of their performances, characteristics and applications. In the light of China's specific conditions of pipeline Industry, the necessity and urgency for developing pipeline internal inspection technology are pointed out and some suggestions given.