Abstract: It is difficult for traditional optical remote sensing technique to extract underlying topography over forest areas because that it can only measure the height of top of canopy. Since the microwave can penetrate into forest and record its vertical information, the microwave remote sensing technique provides the possibility for the underlying topography estimation, which has been a research hotspot. In this paper, the principle of digital elevation model (DEM) extraction by synthetic aperture radar interferometry (InSAR) and the corresponding SAR systems to acquire the InSAR data over forest areas have been introduced firstly. The second part of this paper describes different methods used to extract DEM based on InSAR, polarimetric InSAR (PolInSAR) and tomographic SAR (TomoSAR) technologies. After this, based on the above three SAR technologies, the application and research progress of underlying topography estimation are introduced. Finally, the discussions about the data acquisition, error correction and scattering model reconstruction for underlying topography extraction are introduced.