Inverse problems have a long history in acoustics, optics, electromagnetics, and geophysics, but only recently have the signals provided by ocean acoustic sensors become numerous and sophisticated enough to allow for realistic identification of the ocean parameters. Acoustic signals propagating for long distances in the water column and reflections of underwater sound from the ocean boundaries provide novel problems of interpretation and inversion. The chapters in this volume discuss some of the contemporary aspects of these problems. They provide recent and useful results for bottom recognition, inverse scattering in acoustic wave guides, and ocean acoustic tomography, as well as a discussion of some of the new algorithms, such as those related to matched-field processing, that have recently been used for inverting experimental data. Each chapter is by a noted expert in the field and represents the state of the art. The chapters have ail been edited to provide a uniform format and level of presentation.