Particles are sensed through their interactions with matter. To begin with, the chapter introduces the terms cross section and absorption. Then successively the most important interactions that are employed for the detection of the various particle types are discussed: energy loss of charged particles by ionisation and bremsstrahlung, multiple Coulomb scattering of charged particles, interactions of photons and hadrons with matter. The interactions leading to the development of electromagnetic and hadronic showers are treated in more detail in chapter 15 (Calorimeters), while energy loss by Cherenkov and transition radiation are discussed in chapters 11 and 12, respectively. When describing the interaction processes an attempt is made to address the theoretical background in a way that the derivations ought to be comprehensible.