In this review the basic interaction mechanisms of charged and neutral particles are presented. The ionization energy loss of charged particles is fundamental to most particle detectors and is therefore described in more detail. The production of electromagnetic radiation in various spectral ranges leads to the detection of charged particles in scintillation, Cherenkov and transition radiation counters. Photons are measured via the photoelectric effect, Compton scattering or pair production, and neutrons through their nuclear interactions. A combination of the various detector methods helps to identify elementary particles and nuclei. At high energies absorption techniques in calorimeters provide additional particle identification and an accurate energy measurement. ©2000 American Institute of Physics.