The purpose of pavement-preservation treatments is to correct surface defects, improve ride quality, improve safety characteristics, and extend pavement life without increasing the structural capacity of the pavement. The application of a thin overlay is expected to extend the life of a pavement by 8–10 yr, although this range may vary depending on traffic, environmental conditions, quality of the materials, and workmanship. Thin overlays do not significantly increase the structural capacity of a pavement. Thus, the existing pavement condition should be evaluated carefully prior to the application of a thin overlay to ensure that structural rehabilitation is not necessary. A set of guidelines to determine the best time to apply thin-overlay treatments would help highway agencies optimize their budgets, thereby leading to potentially significant taxpayer savings. The objective of this study was to develop guidelines, parameters, and performance-prediction equations to select the most appropriate time to apply a thin-overlay treatment based on the condition of the existing pavement. To arrive at the proposed guidelines, data from the Long-Term Pavement Performance (LTPP) Program Specific Pavement Studies 3 and 5 were used to evaluate the effects of climate, traffic, existing asphalt concrete (AC)–layer thickness, and overlay thickness on the life extension that results from the application of thin-overlay treatments. The results demonstrate that threshold triggers based on longitudinal cracking in the wheel path and rutting severity can be used to select the best time to apply a thin overlay in order to achieve a target pavement-life extension. Analysis of the LTPP data shows that both the traffic level and existing AC-layer thickness significantly affect the life extension that results from the application of a thin overlay in terms of retarding rutting and longitudinal cracking, respectively. This paper presents empirical equations to predict the life gain that can be achieved from a thin-overlay treatment based on the existing pavement conditions.