The ITER Physics Basis presents and evaluates the physics rules andmethodologies for plasma performance projections, which provide the basis for thedesign of a tokamak burning plasma device whose goal is to demonstrate the scientificand technological feasibility of fusion energy for peaceful purposes. This Chaptersummarizes the physics basis for burning plasma projections, which is developed indetail by the ITER Physics Expert Groups in subsequent chapters. To set context, thedesign guidelines and requirements established in the report of ITER Special WorkingGroup 1 are presented, as are the specifics of the tokamak design developed in theFinal Design Report of the ITER Engineering Design Activities, which exemplifiesburning tokamak plasma experiments. The behaviour of a tokamak plasma is determined bythe interaction of many diverse physics processes, all of which bear on projectionsfor both a burning plasma experiment and an eventual tokamak reactor. Key processessummarized here are energy and particle confinement and the H-mode power threshold;MHD stability, including pressure and density limits, neoclassical islands, errorfields, disruptions, sawteeth, and ELMs; power and particle exhaust, involvingdivertor power dispersal, helium exhaust, fuelling and density control, H-mode edgetransition region, erosion of plasma facing components, tritium retention; energeticparticle physics; auxiliary power physics; and the physics of plasma diagnostics.Summaries of projection methodologies, together with estimates of their attendantuncertainties, are presented in each of these areas. Since each physics element hasits own scaling properties, an integrated experimental demonstration of the balancebetween the combined processes which obtains in a reactor plasma is inaccessible tocontemporary experimental facilities: it requires a reactor scale device. It isargued, moreover, that a burning plasma experiment can be sufficiently flexible topermit operation in a steady state mode, with non-inductive plasma current drive, aswell as in a pulsed mode where current is inductively driven. Overall, the ITERPhysics Basis can support a range of candidate designs for a tokamak burning plasmafacility. For each design, there will remain a significant uncertainty in theprojected performance, but the projection methodologies outlined here do suffice tospecify the major parameters of such a facility and form the basis for assuring thatits phased operation will return sufficient information to design a prototypecommercial fusion power reactor, thus fulfilling the goal of the ITER project.