This study summarizes the experimental and analysis methods used in characterizing and quantifying the internal structure of hot-mix asphalt (HMA) paving mixtures. The implications of the internal structure analysis for the design, compaction, and performance of HMA are also discussed. The methods can be divided into two categories: volumetric analysis methods and imaging methods. The volumetric analysis methods rely on bulk measurements of the percentage of air voids in certain aggregate sizes or in the whole mix as indicators of packing. These concepts are currently used in the Superpave design system for dense-graded HMA, stone matrix asphalt design, and the Bailey method for selection of aggregate gradations in HMA. The imaging methods quantify the distribution of the aggregate skeleton, voids in the mineral aggregate, and air voids by analyzing images of the internal structure. These images can be captured two-dimensionally with a simple setup of a microscope connected to a camera or three-dimensionally with an X-ray computed tomography system.