Highway builders and rehabilitators throughout the United States use asphalt plug joints (APJs) in bridge expansion joints according to manufacturers' recommendations. Joint performance varies widely, as indicated in recent surveys. State departments of transportation are installing joints in unsuitable sites or are overlooking the other sites where APJs may work well. Rational design guidelines for APJs have been developed. The research suggests suitable applications, material characterization, design guidelines, and validation procedures. Two critical material properties are required to qualify APJ material: relaxation and glass-transition temperature. Both properties may be obtained by using a slight modification of the AASHTO standard thermal specimen, restrained specimen asphalt test. This modified standard test was conducted, and the results were compared with those of the near-full-scale test. Design guidelines are based on field observations, material tests, near-full-scale testing, analytical evaluations, and a survey of the experiences of state departments of transportation. Design recommendations are provided, and joint design changes are proposed to help mitigate the present shortcomings.