We present the first systematic ab initio study of cationic defects in a-SiO2. Cluster models are employed to show that the presence of cationic defects (H+, Li+, Be2+, Na+, and Mg2+) can have large effects on both the structure and (hyper)polarizabilities of the material. The interaction of the proton with the cluster model demonstrates that it is unique compared to the other group IA and IIA cations in the study.