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Equilibrium moisture beneath highway pavements is critical to pavement design because it directly affects the strength and stiffness of pavement systems. Moisture is related to soil suction by means of the soil water characteristic curve (SWCC). Previous research has indicated a correlation of suction with Thornthwaite's moisture index and soil type; however, these suction correlations exhibit large variability. Under NCHRP Project 9-23, Environmental Effects in Pavement Mix and Structural Design, sponsored by FHWA, soil samples were collected from beneath 30 pavement sections throughout the United States. The SWCCs and index properties of collected samples were measured at Arizona State University. The in situ degree of saturation was obtained and the corresponding in situ soil suction was found from measured SWCCs. On the basis of the field and laboratory data, two models were developed to predict equilibrium suction under pavements: one for granular non-plastic materials and another for fine-grained plastic materials. These models were adopted in the new AASHTO Mechanistic–Empirical Pavement Design Guide because they exhibited good results, with variability within acceptable limits. Model development is presented.