CsCdBr3 crystallizes in the quasi-linear CsNiCl3 structure. It possesses linear chains of [CdBr6](4-) octahedra separated by parallel chains of Cs+. Trivalent rare-earth (RE) ions substitute for the divalent Cd ions. The need for charge compensation leads to a number of RE centers, the most prominent of which is the symmetric pair center RE3+-(Cd vacancy)-RE3+. Madelung calculations were performed for a number of different centers giving a ranking for their chance of realization. Experimental evidence from optical spectroscopy and submillimeter ESR is given for the most likely centers. The symmetric pair center is of special interest for cooperative phenomena of RE3+ ions. This includes direct ion-ion interactions and interactions with or via the electronic excitations of the host lattice. Some experiments are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.