In this paper we will consider the disorder in some cubic solid solutions consisting of one of the alkaline earth fluorides and one of the rare earth fluorides. This is an attractive group of model materials, because these materials have a rather simple overall cubic structure. We will discuss the most important features of the structure of these solid solutions, which have been obtained using various spectroscopic methods. From these experiments we will conclude that, depending on the radii of the ions in the host alkaline earth fluoride crystal and the trivalent impurities the defect structure varies significantly. In some solid solutions evidence for appreciable preferential clustering has been found. On the other hand there is substantial evidence that for some other materials there is little or no preference towards clustering for impurity concentrations upto even a few mol%; here, the defect structure is determined more or less by statistics.