X-rays are a form of invisible, high-frequency electromagnetic radiation. The wavelength of x-rays is between 10 and 0.01 nanometers, corresponding to a frequency of 30 PHz to 30 EHz. X-rays are very small and energetic. They are produced by accelerating electrons at a metal target. X-rays are used in various medical applications, especially for imaging.

X-rays were a hot topic for research in the late 19th century, and various famous inventors worked on them, including Thomas Edison and Nikola Tesla. Tesla was the first to inform the scientific community of the biological hazards of x-rays. The biggest contributor to x-ray research was Wilhelm Röntgen, who won the first Nobel Prize in Physics for his work. Although the term "x-rays" was only meant to be a placeholder, the name stuck, much to Röntgen's chagrin. In some languages, x-rays are actually referred to as Röntgen rays. His landmark paper in the field was titled "On a New Kind of Radiation."

Because of the tiny wavelength of x-rays, they are useful for imaging small structures, such as individual molecules. This research field is known as x-ray crystallography, which was responsible for uncovering the physical structure of DNA, among other achievements. Today it is frequently used to image the molecular structure of complex proteins.

Sometimes x-ray imaging is used to investigate paintings which have been painted over. Their most prominent use is probably to image bones to discover whether they are actually broken or not. X-rays are a carcinogen, so use of them for medical purposes should be kept brief.

Under special circumstances, such as in a dark room and looking directly down a cathode ray tube, x-rays are visible to the human eye. It is not known whether this is the eye directly perceiving the rays, or from a secondary effect such as the creation of luminescent sparks in the eyeball. In any case, this fact about x-rays is not widely publicized, likely because it would be dangerous to test it very extensively.