In the comparison of frequency standards of different types and of different frequencies to an accuracy of within 1 part in 1010, certain precautions must be taken in the design and operation of the electronic circuits involved.The present system used at the National Physical Laboratory for the calibration of the 100kc/s quartz standards by means of the caesium resonator is described, including an automatic frequency comparator which eliminates certain possibilities of error inherent in other methods of measurement.Brief reference is made to a proposed method for controlling a 5Mc/s quartz oscillator by means of the caesium resonance.