Requirements for collision data on helium, beryllium and boron are reviewed in the light of the directions of present and planned tokamak fusion experiments. The occurrence of the atoms and ions of these species and their roles in plasma behaviour and diagnostic measurements are described. Special emphasis is placed on alpha particle detection in reacting plasmas and beryllium and boron in divertor configuration machines. The atomic reactions required to exploit the species in models and diagnostic analysis are gathered together and their relative importances indicated. The article is intended as an introduction to the detailed studies of collision cross-sections presented in the other works in this volume.