The paper reviews the various ways in which c.c.d.s can be employed in i.r. sensing systems. These include: (i) monolithic structures fabricated using narrow band semiconductors such as HgCdTe or InSb, extrinsic silicon structures doped with deep-level impurities, and silicon Schottky barrier devices; (ii) hybrid structures in which the c.c.d. is used as the read-out mechanism from an array of, for example HgCdTe, PbTe, or pyroelectric detectors. The relative merits of these different approaches are compared and recent experiment results for manystructures are quoted.