In the previous chapter I explained that the common image gathers that can be obtained from downward-continuation migration are more likely to have fewer artifacts, particularly those caused by multipathing, than other migration methods. Unfortunately, in areas where the subsurface is very complex, especially in areas of strong velocity contrasts such as near salt bodies, migration can not provide a complete model due to poor illumination. In this chapter I will explain how to use downward-continuation migration as an operator in an iterative least-squares inversion scheme. I will also explain how I stabilize the inversion by regularizing the output model.