This paper addresses a speckle reduction method of coregistered SAR images based on pixel-to-pixel summation. The sample intensity summation technique (incoherent summation) is well known and is successfully applied to obtain multi-look data. It is shown that the speckle reduction obtained by such a summation depends on the speckle correlation across all the channels. On the contrary, a coherent summation performed by the Whitening Filter leads to an optimal speckle reduction. A comprehensive review of multi-channel summations is presented, followed by practical results obtained from multi-temporal SAR ERS-1 images.