An X-ray is a form of electromagnetic radiation of the same form as visible light, but of an extremely short wavelength (less than 100 angstroms). X-rays have the property of acting on photographic plate to produce images. X-rays can also penetrate solid material. For this reason, X-rays can be used to generate pictures "through" a solid object, e.g. allowing inspection of the contents of a suitcase or visualization of the interior of a patient's body. X-rays also penetrate different materials at different speeds; more dense material shows up as white on an X-ray photograph, while less dense material shows up gray or black. For this reason, X-rays can be used to differentiate bone from soft tissue (allowing assessment of fractures) or normal tissue from diseased tissue (allowing detection of tumors).

X-ray images are two-dimensional, which means that depth information is lost. Also, X-ray photography cannot distinguish between two objects of the same density (both will appear as the same brightness on the photographic image).

Repeated or high-level exposure to X-rays and other radiation can damage cells in the body by damaging DNA and inhibiting the ability of cells' to reproduce. For this reason, targeted exposure to radiation is used to inhibit growth of tumors in some cancer patients.

Current techniques including computed tomography (CT) make use of X-rays to construct detailed images of the interior of the body and brain.