The paper deals with vertical-shaft a.c. generators of large diameter in the speed range 50–500 r.p.m. The influence of turbine and hydraulic characteristics is first considered. This is followed by a discussion of factors which determine the generator dimensions. A brief summary of the considerations affecting rotor design leads to a detailed review of different types of rotor and pole construction and their range of application.An important feature of this type of machine is the thrust bearing and its support, which carry very heavy loads. Alternative types of bearing and bearing arrangement are therefore examined and their merits compared. Methods of bearing cooling are briefly considered.In a Section on the special arrangement of air coolers required for this class of generator, reference is made to the use of generator ventilating air for station heating. This is followed by a Section on the arrangement and operation of brakes and jacks.The stator frame, core and windings; the rotor windings; the exciters and governor generator are briefly considered, and the paper concludes with a short review of special electrical characteristics.