||Future light-based quantum technologies will depend on generation and manipulation of single photons, thus any quantum device will contain one crucial component within – single-photon emitter. So far, among the frontrunners are solid-state single-photon emitters, and they are expected to play an essential role in a plethora of quantum tasks. The recent discovery that hexagonal boron nitride can host robust, narrow-linewidth, bright, photostable, and indistinguishable single-photon sources operating at room temperature and above has sparked interest in this material. This Thesis synthesizes the results of theoretical work on the understanding the role of point defects in hexagonal boron nitride, in particular for the observed single-photon emission, by performing first-principles calculations. A significant step forward in the study was provided by giving a broad and comprehensive picture of the defect chemistry in hexagonal boron nitride. The author of this Thesis ensured a correct treatment of defect physics, and thereby illustrated ability to predict and assign microscopic identity of single-photon emitters in hexagonal boron nitride on scales unreachable for the experiment, using a purely first-principles approach. Physical model for quenching of 2 eV single-photon emission in this material was proposed. The author of this Thesis unpretentiously believes that the study covered in the last chapter of Thesis has also made its small contribution towards a more in-depth understanding of ultraviolet single-photon emission in hexagonal boron nitride by giving convincing explanations for obtained results.