We pointed out in Chapter 4 that speckle in SAR images is a scattering phenomenon. Speckle complicates the image interpretation problem and reduces the accuracy of image segmentation and classification. Multilook processing is a procedure commonly adopted to reduce the noise effect. Speckle filtering or simple averaging can affect the inherent scattering characteristics in polarimetric SAR data. In particular, polarimetric entropy, anisotropy, and averaged alpha angle of Cloude and Pottier target decomposition to be discussed in Chapter 7 requires ensample averaged data for unbiased estimation, and their values are affected by the averaging process. In general, entropy will be underestimated and anisotropy will be overestimated, if the number of looks is insufficiently large.