This paper expatiates on the principles of traditionary median filtering,median filtering based on rough sets,rank-ordered adaptive median filtering(RAMF) and multilevel median filtering.Moreover,the paper reviews application of these filtering principles in enhancing the sonar images from a small underwater target. The experimental results show that traditionary median filtering owns better ability of suppressing sharp pulse and speckle disturbance than the else,but it owns poorer ability of maintaining edges of the images.The experimental results also show that median filtering based on rough sets,RAMF and multilevel median filtering own better ability of maintaining edges of the images than traditionary median filtering,but they own poorer ability of suppressing sharp pulse and speckle disturbance.