This paper outlines the evolution of the hydrogen maser oscillator and describes the basic operational requirements and the present technology. Design features of a high stability maser oscillator for use in space, maser operation at 0.5 K and the present uses of H-masers are discussed.Due to an unfortunate error, the first section (i.e. the Introduction) of the original paper by Dr Robert F C Vessot was not printed in its entirety. The full text of the missing section is printed below, together with the appropriate list of references.