The migration of full wavefields (primaries + multiples) by using the concept of secondary sources at each subsurface grid point is summarized. These secondary sources are two way and contain the grid-point reflection as well as the grid-point transmission properties. They "assist" the primary sources in illumination of the subsurface. Full-wavefield migration is implemented as a closed-loop process in which the estimated reflectivity properties are used to generate all higher-order scattering (multiples and transmission effects), such that the modeled full-wavefield response matches the observed measurements. In addition, velocities can be updated simultaneously. Theoretical considerations and examples lead to the principal conclusion that multiples should be used, not removed.