AASHTO MEPDG-3S, the 2021 supplement to the Mechanistic-Empirical Pavement Design Guide, Third Edition, revises selected chapters of the manual of practice. The supplement updates Chapter 5, Performance Indicator Prediction Methodologies, with a top-down cracking fatigue model based on Paris' Law of crack propagation. That model produces predictions for the area of longitudinal cracking and for average crack depth, extending the guide's treatment of fatigue cracking that initiates at the asphalt pavement surface. Chapter 10, Determination of Material Properties for New Paving Materials, is also addressed in the supplement. It is issued as a supplement to the third edition, so chapters it does not revise remain as published in the base manual. The material is intended for pavement designers and agency engineers who apply the mechanistic-empirical procedure to new and rehabilitated pavement structures.