The effects of hydrogen incorporation in dilute nitride semiconductors, specifically GaAs1-xNx, are discussed. The remarkable consequences of hydrogen irradiation include tuneable and reversible changes in the electronic, optical, structural, and electrical properties of these materials. The highly trapping-limited diffusion of H atoms in dilute nitrides results in the formation of extremely sharp heterointerfaces between H-containing and H-free regions of the crystals. This, in turn, offers an unprecedented possibility to tailor the physical properties of a semiconductor chip in its growth plane with nanometer precision. A number of examples are presented and discussed.