The paper deals with the development of time bases of various types employing hard and soft valves both for general and for special-purpose applications. Since thedevelopmentof time bases is considered it follows that several instruments mentioned herein have been described elsewhere, but the purpose of the paper is to elucidate the principles involved in the wide variety of known time base circuits, rather than to attempt a detailed description of the actual instruments.Except for a few special remarks the application of the time-base potentials and currents to the cathode-ray tube is not discussed, and for this reason the electromagnetic time bases which have been evolved for television purposes are, in general, omitted from the paper. For similar reasons time bases which are solely applicable to high-voltage cathode-ray tubes are not considered.Several devices which are not generally known are described, and the technique of producing and controlling pulses is also considered since this has become of prime importance in many time base applications.The results of a recent investigation into some peculiarities of the gas-discharge triode are presented in the form of an Appendix.