The physics requirements for the ITER design have been set to provide reasonable assurance that the plasma performance will be sufficient to meet the goals of ITER in both physics and technology phases of operation. Considerations for an adequate level of energy confinement with a stable plasma, a satisfactory power and particle control system, an efficient heating and current drive scheme, a suitable plasma control system, a sufficient level of volt-seconds, a need for high-fluence burn, etc., along with several engineering and technology constraints, set the machine parameters (current, size, field, etc.). The physics guidelines for the ITER design are based on reasonable extrapolations of the tokamak physics database as assessed during the ITER Conceptual Design Activities (1988–90). The reference ITER design has a nominal plasma current of 22 MA (with capability up to 25–28 MA for limited pulse duration), magnetic field of 4.85 T, major radius of 6 m, minor radius of 2.15 m, and a plasma elongation (at the 95% flux surface) of ∼2 that produces a nominal fusion power of ∼1 GW.