Much of the Earth's surface is covered by a thin blanket of unconsolidated sediments or weathered bedrock. Boundaries within or at the base of this surficial layer (e.g., watertable, bedrock surface, transition to unweathered rock) are commonly associated with a strong seismic contrast. Layers contained between the surface and shallowest boundaries may be efficient waveguides and, as such, associated with significant source-generated seismic noise. Whereas Rayleigh waves (groundroll) can be described in terms of normal modes, we demonstrate that the faster shingled guided waves can be explained in terms of leaking modes. Due to the relatively high Poisson's ratios in the shallow subsurface, guided waves are composed dominantly of multiply reflected P-waves. We compare our results with those obtained using a conventional acoustic waveguide approximation and discuss methods for extracting information from guided waves for geotechnical and environmental applications. ¿ 1998 American Geophysical Union