Wavefield synthesis is a process for producing reflection responses from more general sources or from prescribed incident waves by combining common-shot data gathers. Synthesis can provide surveywide data sets, similar in that regard to common-offset data gathers, but with the added advantage that each synthesized data set is a solution to a single wave equation. A common-offset data set does not have this last feature. Thus, synthesized data sets can be processed by true-amplitude wave-equation migration. The output is then known to be true amplitude in the same sense as is the output of Kirchhoff inversion. That is, the peak amplitude is proportional to the ray-theoretic reflection coefficient at a determinable specular incidence angle multiplied bythe area under the frequency-domain source signature and scaled by . Alternatively, the Kirchhoff inversion of synthesized data has a Beylkin determinant that is expressed in terms of the ray-theoretic Green's function amplitude. This is in contrast to 3D common-offset inversion, wherein the Beylkin determinant is most difficult to compute. We present a theory of data synthesis and true-amplitude migration/inversion based on the application of Green's theorem to the ensemble of common-shot gathers and prescribed more general sources or prescribed incident waves. Specific examples include delayed-shot line sources and incident dipping plane waves at the upper surface. We also discuss two cases in which waves are prescribed at depth, back-projected to the upper surface, and then used to generate a synthesized data set.