The effective area versus angular resolution challenge: A clear correlation is noted, when plotting the mass required to provide a given effective area as a function of the angular resolving power, expressed as resolution elements per angular unit. The X-ray optics flown to date can be separated into three groups: foil based optics, replicated shells and polished monolithic optics. Although these are very different from each other, the same area/resolution correlation is valid for all of them. The optics for each of the flown missions has been carefully optimised and raised to a high level of maturity. IXO requires a truly novel optics technology, clearly off the correlation line. The IXO optics technology has to be able to provide much more effective area per given mass, whilst maintaining the angular resolution performance than any of the preceding missions to date.