The variations in the frequencies of quartz oscillators compared with that of a caesium standard are described and discussed. It is shown that the frequencies of the ring-type oscillators at the N.P.L and other laboratories do not depart from those given by a linear law of variation by more than ±5 parts in 109over a period of three years. The quartz oscillators calibrated by the caesium standard have been used to study the variations in the period of rotation of the earth. In addition to an annual seasonal variation of approximately ±8 parts in 109, there was a fairly steady retardation amounting to 1 part in 108in the period between September, 1955, and January, 1958.