Although progress has been reported, the extreme-ultraviolet (EUV) source remains one of the largest challenges for EUV lithography production to be economically viable. This article gives an update on the high-level source requirements, including the origin of these specifications. All requirements are driven by litho system cost-of-ownership considerations and the imaging capabilities. Attention will be paid to conflicting requirements and the consequent tradeoffs that have to be made in making conceptual source decisions and in designing an EUV system. Finally, we will look into the future of the top-level source requirements with the conclusions that not only EUV power requirements will keep on increasing, but that this increase has to be accompanied with a high availability and reliability of the source. Furthermore, an operation mode has to be found that is chemically, particulately, and spectrally clean so that the cost-of-ownership and imaging capabilities of the total system remain intact over its lifetime.