Among the important early researchers in X-rays were Sir William Crookes, Johann Wilhelm Hittorf , Eugene Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von Helmholtz, Thomas Edison, Nikola Tesla, Charles Barkla, and Wilhelm Conrad Roentgen.

Physicist Johann Hittorf observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugene Goldstein. Later, English physicist William Crookes investigated the effects of energy discharges on rare gases. He constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he places unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect. In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminum). Philip Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration of X-rays through various materials. Philip Lenard, though, did not realize that he was producing X-rays.