This article summarises the constraints for tokamak operation. The operating space is restricted by several limitations among which the plasma performance and hence the resulting fusion power has to be optimized. Hard limits which lead ultimately to a disruption and may damage the first wall as well as soft limits resulting in a reduction of the energy content (and the available fusion power) of the plasma are described. The operational limits can be summarized in two general groups: excessive radiation from the plasma, and violation of global as well as local MHD stability boundaries.