The volume describes the optical spectra of Nd2+ and Nd3+ ions in host materials. The Nd3+ ion is one of the most important activator ions in solid-state laser crystals. Thus its optical spectra arising from transitions within the levels of the 4f manifold have been extensively investigated. The field is surveyed by an expert author who discusses the spectra of Nd3+ in solid inorganic neodymium compounds in great detail. In addition, a complete survey is given on the Nd3+ spectra in host crystals of other inorganic non-rare earth compounds and also on the effects of codoping of additional rare earth ions in these compounds. Spectra of Nd3+ in other hosts will be discussed in a forthcoming volume.