Scintillation and semiconductor detectors and their properties are described in detail. Some of the most useful detectors such as NaI(Tl), BGO, LSO, LYSO, and CZT have specific applications in detecting radiations, which are discussed. Intrinsic and geometric efficiencies and energy resolution are defined and calculated. The interaction of radiations in NaI(Tl) crystal is elaborated. The design of well counters, liquid scintillation counters, and thyroid counters and their operation and utilization are elucidated. The dead time of different counters are reported.