This chapter firstly introduces the properties of detectors and presents a synoptic discussion of the organization of the following chapters. The single detectors will be the building blocks of the astroparticle instrumentation discussed later. Detectors for high energy photons and particles show a great difference compared to optical and radio detectors, the intrinsic energy discrimination capability coupled to the imaging capability. The chapter ends with a summary of some historical detectors in astroparticle physics.