Speckle reduction is always one of the important tasks in the application of SAR(Synthetic Aperture Radar) images . With the development and application of SAR

many speckle filtering algorithms have been proposed aimed at speckle reduction in lately twenties of years. These algorithms can be divided into two categories: one is based on single channel data

and the other is based on multi channel data. Comprehensively and systemically

this paper investigates the principles of speckle reduction in polarimetric SAR images. All of the algorithms proposed in recent years are summarized and their performances are analyzed and compared.