Abstract: A comprehensive summary is given of the Magnetotelluric Sounding, for which the nature Earth's electromagnetic field is used as source, and there is large penetration depth. This is an effective approch particularly to survey structure and state of matter in the deep Earth, so possesses good prospect. At first the character of Earth's electromagnetic field and the basic principle of the method is presented, afterwords several breakthrough characteristic advances that have put forward since the early 1960's is reviewed, and finally the survey results in a number of significant application are discussed in detail.

Key words: magnetotelluric sounding, crust, mantle, conductive layer, resistivity