The physics knowledge relevant to the design of a reactor-scale tokamak—the ITER Physics Basis—has recently been assessed by the ITER JCT, the ITER Home Teams, and the ITER Physics Expert Groups. Physics design guidelines and methodologies for projecting plasma performance in ITER and reactor tokamaks are developed from extrapolations of various characterizations of the database for tokamak operation and of the understanding that its interpretation provides. Both “conventional” and “advanced tokamak” operating modes are considered. The overall device parameters for ITER are found to be consistent with these guidelines. The plasma performance attainable in ITER is affected by many physics issues, including energy confinement, L-to H and H-to-L-mode power transition thresholds, MHD stability/beta limit, density limit, disruptions, helium removal, impurity content, etc. Design basis and guidelines are provided in each of these areas, along with sensitivities and/or uncertainties involved.