The light source is a critical factor when Extreme Ultra Violet Lithography (EUVL) is used. This paper shows the link between the light source and the customer's requirements. These are: Throughput, Cost of Ownership (CoO) and imaging quality (e.g. CD uniformity). Also shown is how customer requirements dictate the necessary performance specifications for light sources. To be competitive with other New Generation Lithography technologies, EUVL tools should show a potential for high throughput. This puts a high requirement on the collectable in-band power produced by an EUV source. CD control requirements, together with restrictions of reflective optics, are discussed. This means that the pulse-to-pulse repeatability, the spatial stability control and the repetition rates, must be substantially better than those of current optical systems. It is essential, in the early stages of the development of potential light sources, to understand the source limitations. It is also essential to see the way the light source or the total source/lithographic system could be improved, in the development time, to meet the lithography tool requirements. Although there are no light sources currently under development that comply with the requirements for an EUVL production tool, future improvement in the performance of light sources seems possible. This paper shows detailed analyses of the way to meet (beta) and production tool requirements, including the increase in repetition rate, for all of the sources. The decrease in energy per pulse will also be discussed. The performance requirements for the light source for the first lithographic tool, together with the timeline for its choice, will be shown.