In this chapter, I review modeling and migration by recursive wavefield extrapolation, contrasting the exploding-reflector, shot-profile and shot-geophone methods. I then show that for the special case of sparse-shot geometries with fully sampled receiver axes, the main difficulty with obtaining accurate migration amplitudes in vertical incidence reflections is the variability in shot illumination. I show that shot illumination problems can be compensated for by a model-space weighting function applied after migration. The weighting function is based on integrating the total downward-going energy at each point in the subsurface, which can be calculated at very little cost during the migration process.