[en] Single crystal X-ray diffraction (XRD) is a powerful non-destructive method which allows unambiguously identify crystalline phases, determine a crystal structure (unit cell parameters, a space group, atomic coordinates and atomic occupancies) and, if required, a phase composition. This thesis deals with applications of single-crystal XRD in high pressure and high temperature (HPHT) research using laser-heated diamond anvil cells (DACs). The thesis describes methodological aspects of our single-crystal XRD experiments which involve crystals selection, DACs preparation, maintaining experiments, data processing, and structure solutions and/or refinements. We demonstrate a great potential and novel opportunities provided by high-pressure crystallography in materials- and geo-sciences on the examples of studies of transition metal borides, a metal-doped boron phase, silicates, and oxides, particularly refined crystal structures of Co₅B₁₆, MnB₄, Al-doped β-boron, knorringite, and Fe³⁺-bearing bridgmanite, investigated the high-pressure behaviour of FeB₄, Fe₂B₇, Fe_xB₅₀, and FeOOH. A unique atomic arrangement in the FeB₄ brings it to a class of superhard materials with a nanoindentation hardness of 62(5) GPa. We found that the structure of Fe_xB₅₀ composed of B12 icosahedra has large cavities, so it can contract more effectively than boron polymorphs (α-, β- and γ-boron), also containing chemically bonded B12 icosahedra. The distribution of iron in Al-free, Fe³⁺-bearing Mg-perovskite (bridgmanite) was derived from single-crystal XRD combined with Moessbauer spectroscopy. High-pressure and high-temperature (HPHT) single-crystal XRD was used to search for HPHT polymorphs of Fe₂O₃ and Fe₃O₄ in a megabar pressure range and to uncover the fate of the iron oxide in subducted banded iron formations (BIFs) in the Earth's lower mantle. The authors confirmed that above 29 GPa Fe₃O₄ adopts the crystal structure of CaTi₂O₄ which is stable to at least 70(1) GPa and 2400(100) K. The authors have resolved the over 50-year old controversy regarding the structure of the Fe₂O₃ polymorph stable above ∝50 GPa. Thus, the Fe₂O₃ from subducted BIFs may be a source of an oxygen-rich fluid to the deep Earth's interior with significant amount of oxygen (up to 8 times the amount of oxygen in the modern atmosphere), leading to significant heterogeneity in oxygen fugacity in different parts of the mantle.