Charged particles passing through an appropriate path length of material will ionize the atoms. We saw, that the average energy loss of heavy charged particles is determined by the Bethe-Bloch equation (see ( 2.13 )) and the most probable energy loss is given by ( 2.40 ). Thus, ionization measurements play a basic role for charged-particle detection. The simplest application discussed in this chapter is the usage of ionization measurements in detectors that record radiation. This is used to localize radioactive sources and measure their activity or to localize residual radiation and its intensity in accelerators. As we will see, the measurement of different particle properties depend on ionization measurements, such as identifying trajectories and vertices, measuring electromagnetic or hadronic energy and identifying particle types at low momenta. For example, from the measured particle energy loss we obtain the particle velocity as discussed in Chap.  2 . Thus, tracking and vertex detectors provide particle identification in the low-momentum region in addition, if the particle momentum is known. This will be discussed in detail in Sect.  9.1 .