Designers of low-power radio systems for use in urban areas would benefit from the capability for accurate computer-based predictions of signal loss due to shadowing. This thesis is intended to fill a need for prediction methods that exploit a building database and consider the three-dimensional profile of the radio path. Models are presented that allow the application of Fresnel-Kirchoff diffraction theory to arbitrarily oriented buildings of simple shapes. Building location information used by the diffraction models is in a form compatible with a geographic information systems (GIS) database. Diffraction screens are constructed at all building edges, including those of both horizontal and vertical orientations, in order to consider all possible diffractions and to compute field contributions often ignored. Multiple buildings and edges of the same building that introduce multiple successive diffractions are considered with a rigorous, recursive application of the diffraction theory that requires sampling the field distribution in each aperture. Robust and computationally efficient numerical methods are applied to solve the diffraction integrals. The software implementation of these methods is tested with example runs and comparisons with 914 MHz continuous-wave measurements taken on the Virginia Tech campus. / Master of Science