It is now widely accepted that DX levels in n-type AlxGa 1-xAs are ground states of isolated substitutional donors in distorted configurations which are stabilized by trapping two electrons. The leading model is that the distortion occurs when the group IV donor substituted on a group III lattice site, moves along a <111> axis towards an interstitial site. In the case of a group VI donor which substitutes on a group V lattice site, one of the group III neighbours moves toward the interstitial site. Recent work suggests that localized excited states of both donor configurations play an important role in electron transitions between DX levels and the conduction band. The research leading to this view of substitutional donors is reviewed. Some implications of this model for technological applications of AlxGa1-xAs and related alloys are discussed.