In this year, we celebrate the 100 years anniversary of Arthur Wehnelts discovery of the oxide cathode. In his famous article in "Annalen der Physik" in 1904 Welmelt showed that coating of noble metals with alkaline earth oxides, such as BaO and CaO lowered cathode fall and work function drastically. This was the start of the oxide cathode, which has become one of the most commonly used electron sources in vacuum tube technology and also in low pressure gas discharge lamps. In the past century, a vast amount of literature has been generated on the technology and emission mechanisms of the oxide cathode. This paper reviews the evolution of technology, applications and theoretical models of the oxide cathode from 1904 to the present status. (c) 2005 Elsevier B.V. All rights reserved.