Seismic reflection techniques are the primary geophysical tool used to examine the structure of continental margins. On convergent plate margins, widely separated seismic reflection profiles often do not image complex structural features because out-of-plane reflections and diffractions obscure the seismic images. Yet the structural features are often critical to understanding the tectonic processes. Thus, the resulting geologic interpretations are based on images that may not accurately represent the true subsurface structure. To image the complex geologic structures of an active continental margin, a three-dimensional (3-D) seismic survey was conducted off of the Nicoya Peninsula of Costa Rica in April 1987. Over 60,000 shot points of 96-trace multichannel data were collected in a 9×22 km grid. This detailed survey was located over the accretionary prism and designed to investigate the active tectonic processes and evolution of this continental margin. In this paper we outline the data acquisition program, including the navigation processing critical to successful imaging, and the seismic processing methodology and then show several examples of images improved by 3-D surveying. The resulting 3-D seismic data were migrated both as independent lines and as a 3-D volume. Seismic images were produced as conventional depth and time sections and as cross-sectional slices in depth and time. We present comparisons of the migration results that show significant improvements of the subsurface image using 3-D techniques. To illustrate these improvements, we present images of two complex geologic areas: a mud volcano and reflections near the top of the accretionary prism, and the top of the subducting oceanic basement beneath the prism. Comparisons between the original stacked data, the data of the two-dimensional migration, and the results of the 3-D migration illustrate the value of 3-D techniques in studies of active margins. In addition to the advantages of the 3-D imaging which include better structural delineation and improved signal-to-noise ratio in the final image, we show that the ability to view the 3-D data volume in both section and plan view offers significant interpretational advantages.