X-ray windows are used in sources and detectors to separate the neighborhood of the x-ray generation or detection from the use environment. While each use has its own requirements, there are some principles that should be used in designing an optimal x-ray window. Because x rays are absorbed to some extent by all materials, minimizing absorption is one criterion in preparing windows. Also, for most uses there is a pressure difference across the window so that ensuring pinhole-free structure and sufficient mechanical strength to support the differential is another criterion for window design. Traditionally, absorption, is minimized by fabricating the window with lower atomic number (low Z) elements such as Be, B or C. However, the wavelength (energy) region of interest becomes a very important complicating factor.