The JET 32 MW ICRF system is modular in construction and consists of eight independent generator-antenna units operating at frequencies in the range 23-57 MHz. The system is controlled remotely, and all real time functions, such as the matching to the plasma load, are performed automatically during the tokamak pulse. The flexibility and versatility of the plant have successfully permitted a continuing development programme since the system commenced operation in 1985. The total power of the eight generators has been upgraded from 24 to 32 MW, of which over 22 MW has been coupled into the plasma centre. The plant and power availability has been considerably improved by enhancing the automatic matching and protection circuitry. The operating regimes have been extended beyond the original conceptual design, making it possible to heat during L to Fl mode transitions and perform preliminary Fast Wave Current Drive experiments. New facilities will enable phased conductor FWCD experiments with the four-element (A2) antenna arrays during the pumped divertor phase of JET. This paper reviews the enhancements to the plant since its inception, especially to the control circuitry, which have resulted from analysis and understanding of the fast varying antenna-plasma load and from operating under increasingly more stringent conditions. References to the associated ICRH physics are given. The developments of the JET antennae are described in the companion paper ''Present and Future JET ICRH Antenna'' by A.S. Kaye.
