An extensive experimental investigation of reactor-relevant features of H-mode plasmas has been carried out using the JET-pumped divertor. Steady-state H-modes have been established with high confinement and with durations of up to 20 s. At the highest densities, these plasmas return to the L-mode when the radiated power fraction is %. Detached H-modes have been produced by gas-puffing deuterium plus nitrogen, and their properties studied at total input powers of up to 32 MW. Helium transport and exhaust have also been investigated, and a systematic comparison of H-modes using carbon fibre composite and beryllium divertor targets has been carried out.