The recent revolution in computational capability, combined with finite-element analysis techniques, has greatly expanded the ability to analyze vibrational modes of simple and complex structures and acoustic radiation from these structures. These developments have encouraged belief that one may be able to categorize or classify complex structures of interest to the U.S. Navy (e.g., submarines, mines, or swimmers) by analysis of the acoustic energy reflected and reradiated from such structures (echo structure). A detailed discussion of this topic is beyond the scope of this meeting. However, academic researchers wishing to obtain support for their research may benefit from some discussion of the importance of practical limitations such as finite transducer bandwidth, multipath propagation, and population densities on the relevance of classification methods for naval purposes. Some of these issues will be discussed in this presentation.