When Röntgen discovered X-rays he was investigating the conduction of electricity in gases at low pressures, i.e. gas discharges. He was not alone in making such studies; Sir William Crookes, for example, after whom the Crookes tube was named, was a leading scientist in this field. However, gas discharge is not necessary for the production of X-rays: the requirement is for high speed electrons to be stopped or slowed suddenly. The illustrations in this chapter form a small selection of the wide variety of X-ray tubes available in the gas tube era before the developments by William Coolidge provided the enormous improvement in X-ray production which was made possible by the hot cathode tube (see Chapter 7), which in turn made the gas tubes obsolete. The captions to the illustrations provide details of some of the design developments of gas tubes in Europe and in America.