Wave-equation migration velocity analysis is derived from wavefield-continuation migration techniques. The velocity model is updated by optimizing certain properties of the migrated images. Different migration velocity analysis optimization criteria exist, of which two commonly used are fitting a target image and minimizing the differential semblance of migrated images. Both techniques are special cases of a general family of optimization functions. Fitting a target image is an attractive technique because we can guide the solution in the desired direction. However, we can only progress in small steps with the target image being kept within the Born approximation with respect to the reference image. Minimizing differential semblance is an attractive technique, too, because we are operating with small differences of nearby offsets which are likely within the Born approximation. However, this method is not directly guided toward the solution and aliasing or any remnants of coherent noise, like multiples or converted waves, can cause it to diverge.