This chapter is dedicated to reaching deep and full awareness regarding intrinsic defective engineering of Ta₂O₅, Ta₃N₅, and TaON. In the beginning, nature and basic features of defects are addressed in order to understand their origin, nature, and consequent influences on the changes of specific characteristics and properties of these materials. It is followed by the discussion regarding the methods and approaches which can be used to realize their controlled generation with defined type, concentration, and distribution. Special attention is given to thermally activated nitridation process as it is considered to be the one of the main synthesis courses to obtain non-stoichiometric Ta₃N₅ and TaON. Finally, it is demonstrated how the presence of defects can be correlated with photocatalytic performances of these materials toward application in water splitting processing, water purification via dye degradation, and CO₂ reduction.