The performance and lifetime of industrial tools in mechanical engineering, optical or electronic applications is highly influenced by their specific working conditions, e.g. mechanical load, environment and especially temperature. The application of nanostructured functional coatings for high temperature tasks has thus evolved as a cutting edge technology, e.g. on tools in cutting industry, components in automotive industry, as well as in the field of electronics like diffusion barriers, heat sinks or light-emitting diodes. Especially transition metal (TM) aluminium nitrides devolve increasing industrial interest, as they exhibit outstanding physical and chemical properties under thermal load, such as good oxidation resistance or increasing hardness with increasing temperature, known as age hardening. In order to assess the atomistic origin of the age hardening phenomenon in a first step, analytical tools such as atom probe tomography, high resolution transmission electron microscopy, nanonindentation or X-ray diffraction were applied to study the structural evolution of metastable Ti1-xAlxN solid solutions as a function of thermal load. It is shown, that age hardening of cubic Ti1-xAlxN originates from the formation of a three-dimensional interconnected network of cubic TiN- and AlN-enriched domains with increasing temperature due to spinodal decomposition, which results in increasing coherency strain and thus hindering of dislocation motion. At sufficiently high temperatures, or lower temperatures but longer times, respectively, the cubic AlN-rich domains transform into the stable w-AlN modification which implies the development of a dual phase structure with deteriorating mechanical properties at higher temperatures. In the second step, this work presents a combination of quantum mechanical calculations (density functional theory, DFT) with experiments, aiming for an enhancement of the thermal stability of cubic Ti1-xAlxN. The effect of TM-element alloying on structure, mechanical and thermal properties of Ti1-x-yAlxTMyN, with Al-contents close to the metastable cubic solubility limit, is experimentally studied and compared to the ab initio predictions. The main focus of this work was to elucidate the complex processes during phase formation of cubic Ti1-x-yAlxTMyN (TM = Y, Hf, Nb or Ta) by physical vapour deposition and further the phase evolution of metastable cubic Ti1-x-yAlxTMyN solid solutions as a function of annealing temperature. It is shown, that the alloy stability strongly depends on the electronic structure, lattice strain and activation energy for diffusion at high temperatures. The experimental results exhibit an excellent agreement with the ab initio predictions and indicate a profound increase of the thermal stability for already small Ta-additions of ~1.5 at.%. However, alloying with Hf and Nb requires 2.5 to 5 at.% to yield nearly comparable results, while similar additions of Y result in the formation of hexagonal Ti1-x-yAlxTMyN, with detrimental mechanical and thermal properties.