  Various techniques are used to detect the presence of charged particles
stored in electromagnetic traps, their energy, their mass, or their internal
states. Detection methods can rely on the variation of the number of trapped
particles (destructive methods) or the use of the ion's interaction with
electromagnetic radiation as a non-destructive tool to probe the trapped
particles. This review gives an introduction into various methods, discussing
the basic mode of operation completed by the description of recent
realizations.
