Mechanistic design concepts for conventional flexible pavement (asphalt concrete (AC) surface + granular base/subbase) for highways are proposed and validated. The procedure is based on ILLI-PAVE, a stress dependent finite element computer program, coupled with appropriate transfer functions. Two design criteria are considered: AC flexural fatigue cracking and subgrade rutting. Fatigue cracking is controlled by limiting the tensile strain at the bottom of the AC layer. Subgrade rutting is controlled by limiting the stress-ratio at the granular layer-subgrade interface. Algorithms were developed relating pavement response parameters (stresses, strains, deflections) to AC thickness, AC moduli, granular layer thickness, and subgrade moduli. Extensive analyses of the AASHO Road Test flexible pavement data are presented supporting the validity of the proposed concepts.