The principles underlying the design of the 30-MeV synchrotron are described in relation to general design principles for machines up to 400 MeV. The mechanism of operation of electron synchrotrons is described, and formulae are derived for specifying design parameters. Some practical points in the design of synchrotron magnets, accelerating chambers and electronic equipment are then considered. Finally, theradiation characteristics of the machines are described. The paper isillustrated by experiments on the 30-MeV A.E.R.E. synchrotron, and by design data on the 375-MeV Glasgow synchrotron.