Mechanistic-empirical (M-E) pavement design procedures such as the AASHTO 2002 design guide have been widely accepted and are being implemented by many highway agencies. However, several issues need to be resolved before this implementation is completed. One issue is that the design procedure should provide a consistent pavement performance level considering inherent variability associated with design input parameters. For a complete M-E pavement design procedure, the effects of the inherent variability of input design parameters on predicted pavement performance must be addressed and quantified. Some key principles for applying the reliability concept to M-E pavement design are presented to account for the inherent variability of design parameters. In particular, an appropriate reliability assessment technique and careful characterization of design input variability were considered because of their central role in calculating the reliability of pavement performance. Finally, a practical probabilistic design format for evaluating uncertainties in the M-E flexible pavement design procedure is introduced.