Nanostructured metal oxides are being intensively studied for various advanced applications, such as the conversion of solar radiation into other forms of energy, energy storage in Li-ion batteries, adsorption and photocatalytic degradation of harmful substances, detection of harmful and flammable gases or improvement of the properties of polymer materials through the preparation of composites. Due to their strong absorption of UV radiation, significant absorption of visible and solar radiation, chemical stability, corrosion resistance, low cost and environmental suitability, nanostructured iron oxides are suitable materials for these applications and have recently been increasingly investigated. The most prominent research is the possible application of nanostructured hematite (α-Fe2O3) as a photoactive material for hydrogen production by photoelectrochemical water splitting or photocatalytic degradation of harmful organic compounds. The physicochemical properties of iron oxides and their performance in various applications depend on the size and shape of the particles, doping with metal cations and combination with other materials (metal oxides, carbon nanomaterials, polymers) into functional composite materials. Polymer composites are materials consisting of a polymer matrix (continuous phase) and a filler (discontinuous phase), the combination of which results in the formation of materials with unique properties that differ from those of the constituent components. Polymer composites with iron oxides as fillers represent new materials that can have improved mechanical, thermal, barrier and/or UV blocking properties and are non-toxic and safe for use. In this project, nanostructured iron oxide particles of various sizes and shapes will be synthesized using hydrothermal methods by adjusting the synthesis conditions (reactant concentration, temperature, pH, reaction time) and adding shape-directing agents (anions, cations). The influence of particle size and shape and the incorporation of metal cations on the structural, magnetic, optical and photocatalytic properties of the prepared samples will be examined. The synthesized iron oxide particles of various sizes and shapes will be used to prepare polymer composites with biodegradable polymers polycaprolactone and polyvinyl alcohol. A review of the scientific literature has shown that the preparation and characterization of iron oxide composites with individual polymer matrices has been carried out so far, while the polymer matrices polycaprolactone and polyvinyl alcohol have not been systematically investigated. It is important to emphasize that there are no systematic studies of the influence of hematite particles of various sizes and shapes on the properties of polymer composites. The importance of this research lies in the use of biodegradable and biocompatible polymer materials and hematite as a filler, which is iron oxide with properties of non-toxicity, thermal stability, corrosion resistance and strong absorption of UV radiation.