Abstract <p>This study presents a review and comparative analysis of existing approaches, methodologies, and findings related to the thermal conductivity of nanostructured solids. The authors developed theoretical models to predict the effective thermal conductivity of nanopolycrystals and nanocomposites with imperfect grain boundaries, incorporating the effects of isotopic composition, crystal size, interfacial thermal resistance, and temperature. These models rely on the current understanding of the physical mechanisms of lattice heat transfer in covalent crystals and phonon scattering at structural defects. The developed theory offers a straightforward method for estimating the thermal conductivity of nanocomposites and provides insight into the dominant factors governing heat transfer in crystalline structures at the nanoscale. Comparison with experimental data confirms the model’s validity and its applicability to the prediction of thermal conductivity in real-world nanopolycrystalline and nanocomposite materials, including those containing diamond.</p>