The use of ground-penetrating radar (GPR) for pavement investigation has developed rapidly over the past 20 years. The technique involves recording the passage of electromagnetic pulses transmitted into the pavement structure. GPR has enhanced and improved the range and certainty of information that can be obtained from pavement investigations. Analysis of data can provide information on layer depths, material condition, moisture, voiding, reinforcement, and location of other features. The dielectric constant is a material property that affects the speed and reflection amplitude of electromagnetic GPR pulses. Accurate determination or estimation of the dielectric constant is required for accurate analysis of pavement material information from GPR data. Typical pavement materials will have a bulk dielectric constant used in analysis that is the result of both the material constituents (binder, aggregate, etc.) and condition (moisture content, amount of voiding, etc.). This paper aims to provide a review and assessment of in situ dielectric constants of bituminous pavement materials determined from analysis of GPR data. The results of a large number of in situ pavement investigations, on a range of bituminous materials of varying condition, are reported. Dielectric constants from analysis of GPR investigations are determined and compared with existing data, and the effect of material condition and properties are discussed and assessed. The paper concludes that improved assessment of the in situ dielectric constant can be conducted and provide enhanced information from radar data analysis if consideration of material condition is made when selecting the values used in the analysis.