Some experimental applications of positive and negative ions in probing the properties of superfluid helium are described. Particular emphasis is placed on the breakdown of superfluidity, which can occur either through roton creation or by the production of quantised vortex rings. Roton creation experiments have enabled the Landau critical velocity to be determined precisely for the first time and have also shown that rotons are apparently produced through a pair production mechanism. Vortex creation experiments provide a unique source of information about the nature of the microscopic nucleation mechanism.
Invited paper at the 4th General Conference of the Condensed Matter Division of the European Physical Society, Den Haag, 19-20 March 1984.