  Fast particle-driven waves in the ion cyclotron frequency range (ion
cyclotron emission or ICE) have provided a valuable diagnostic of confined and
escaping fast ions in many tokamaks. This is a passive, non-invasive diagnostic
that would be compatible with the high radiation environment of
deuterium-tritium plasmas in ITER, and could provide important information on
fusion {\alpha}-particles and beam ions in that device. In JET, ICE from
confined fusion products scaled linearly with fusion reaction rate over six
orders of magnitude and provided evidence that {\alpha}-particle confinement
was close to classical. In TFTR, ICE was observed from super-Alfv\'enic
{\alpha}-particles in the plasma edge. The intensity of beam-driven ICE in
DIII-D is more strongly correlated with drops in neutron rate during fishbone
excitation than signals from more direct beam ion loss diagnostics. In ASDEX
Upgrade ICE is produced by both super-Alfv\'enic DD fusion products and
sub-Alfv\'enic deuterium beam ions.
