This paper analyses the problem of motion estimation from a sequence of stereo images. Both the differential and discrete approaches of two methods are formulated The differential approach uses differential optical flow whereas the discrete approaches uses feature correspondences. Both methods are used to compute, first, the 3D velocity in the depth (Z) direction and, second, the complete rigid motion parameters. Furthermore, the uncertainty propagation models for both methods and approaches are derived These models are analysed in order to point out the critical variables for the methods. The methods were extensively tested using synthetic images as well as real images and several conclusions are drawn from the results. The real images are used without any illumination control of the scene in order to study the behavior of the methods in strongly noisy environments with low resolution depth maps.