In this paper, we present recent results of our studies of the defect characteristics in two specific materials systems: Cd- and Zn-based II-VI compounds, where we are interested in the influence of growth and doping conditions on the defect microstructure, and wide band gap oxides, which are used extensively for applications as luminescent phosphors, especially when doped with rare earth atoms. It is shown that positrons are excellent probes for sintering-induced crystallization processes and can be used to explore the incorporation characteristics of certain dopants.