In order to comprehend the functions of tertiary mirror applied in large aperture telescopes and meet the requirements for developing large aperture telescope, the technologies of tertiary mirror systems are summarized from the development of optics system configuration, variation of aperture and variation of functionality. A more detailed description about the tertiary mirror technology is given with the conceptual design of Thirty-Meter Telescope (TMT) tertiary mirror system. The tertiary mirrors play a more and more important role in telescopes, especially in large aperture telescopes. With the development of telescope technology, the requirements for tertiary mirrors also become more and more strict, and the functions of tertiary mirrors become more and more variable.