The evolution of the hydrogen thyratron from its early beginnings to the present day is presented from the perspective of one manufacturer. Radar requirements drove the development of the hydrogen thyratron as a repetitive, high power line-type modulator switch. Before the invention of the hydrogen thyratron, line-type radar modulators used a variety of switches. It is worthwhile considering one of these in particular, the mercury vapour thyratron, because this device contained many of the necessary design features that were to be incorporated into the hydrogen thyratron.