The Charge Injection Device (CID) is an x-y addressed solid state imager introduced by General Electric in 1973 as a television sensor. A family of sensors has evolved to exploit the singular CID characteristics and capabilities. This paper describes the structure and principle of operation of a typical CID. A TV compatible differential current sensing mode of operation is described. A multiple non-destructive readout mode is described with examples of application to transform code generation and centroid interpolation. The CID radiation hardening potential is briefly reviewed.