The last 20 years have seen a tremendous amount of work in the area of reverberation and scattering. An effort is made to summarize with graphs and charts the amount and nature of this work. Comparisons are made between the results of measurements at sea, measurements with models, and theoretical calculations. Since theoretical solutions of scattering from randomly rough surfaces involve numerous approximations, these different solutions are classified with the aid of tree diagrams so that one can chart a path through the various approximations. Most of the unclassified work on echo structure has been devoted to intensive analyses for targets of relatively simple geometric shape. For example, the sphere and the cylinder (with and without internal structure) still present interesting problems.