Freestanding diamond foils find applications as windows for high-energy radiation windows. For example carbon foils interact with energetic ion beams by stripping of shell electrons without significantly slowing down the ions in accelerator physics experiments. Foils and windows differ in thickness, i.e. foils are several microns thic whereas windows can be as thick as a millimeter. The foils and windows are made from a range of diamond materials including nano- and microcrystalline doped and intrinsic variations. While being only a few microns thick the foils are mechanically rigid maintaining the pressure gradient from atmosphere to vacuum. Thus X-ray and electron beam sources operating in vacuum can use diamond foils as windows to direct the beams to the outside of the source.