Cuprous iodide has been investigated since 1907 when Karl Bädeker prepared this material from metallic copper thin films with subsequent iodization and reported it as fully transparent conductor. Nowadays CuI is recognized as p-type wide bandgap, transparent semiconductor, offering rather high hole mobilities of so far up to 10 Vs∕cm2 in thin films. The charge carrier density is primarily controlled via the amount of copper vacancies. CuI has been prepared as bulk material and substrate and thin film as well as in the form of various nanostructures. Thin films can be prepared by various techniques including iodization of copper and by thermal evaporation, sputtering or pulsed laser deposition of CuI. Recent progress is represented by the epitaxy on other semiconductors, in particular zinc oxide. CuI has found use as intermediate layer between ITO and organic absorbers in solar cells. Recently, bipolar heterostructure diodes prepared from p-CuI∕n-ZnO layers on sapphire were found to exhibit very high rectification. This makes CuI interesting for use in transparent electronics. For further details see the Review Article by M. Grundmann et al. on pp. 1671–1703.