This paper provides an overview of the state-of-the-art of GPR technology as applied to the measurement of as-built conditions, with specific application examples and supporting research study results. Knowledge of the as-built conditions is an essential part of any nondestructive evaluation program. Existing records and drawings often do not represent what actually took place during construction, nor do they represent changes and repairs that have taken place after initial construction. Ground penetrating radar is a powerful NDE tool for the measurement of the internal dimensions of constructed materials. Existing capabilities include the determination of the thickness of asphalt, concrete, and granular material layers, and measurement of the depth and location of reinforcing steel. The equipment technology has been developed for data collection at up to normal traffic speeds (on pavements and bridge decks), and analytic techniques have been developed to automate the data processing. The paper will describe the status of the GPR equipment technology that is available for this application, including equipment features, data acquisition methodologies, and data samples for different applications. The paper will also describe the basis principles for GPR data interpretation. Based on these principles, software has been developed to automate the processing of pavement layer thickness and depth and location of reinforcing. The software will be described, and example results from these applications will be provided. Results of research studies which have investigated accuracy of GPR based on comparisons to standard cores will also be presented.