Both oxygen deficiency and excess are possible in pure and doped bariu
m titanate and do influence in a passive or active manner its properti
es. Their crystal engineering by gas reduction and/or oxidation is rev
iewed by ''high-lighting'' the aspects of chemical and crystallographi
c properties, dc-field degradation, electrooptics, and thin layer tech
nology and plasma treatments. As a conclusion future trends in barium
titanate technology and applications are mentioned.