In this PhD thesis the microstructural design of AlxCr1-xN thin films (0 <x <1) for high temperature applications is investigated. The use of physical vapor deposition techniques enables the synthesis of single- and polycrystalline films in metastable solid solutions. Simultaneous thermal analysis, combining differential scanning calorimetry, thermogravimetric analysis, and mass spectrometry, together with x-ray diffraction and analytical transmission electron microscopy are used to study ongoing reactions up to 1500 °C in different atmospheres. Subsequent nanoindentation allows correlation of mechanical properties, like hardness, with microstructural developments in the material. A general phase transition from cubic rock salt (B1) to hexagonal wurtzite (B4) structure is observed for x>0.7, however, in this work the cubic structure could be stabilized up to x=0.81 by epitaxial growth. The addition of Al improves the oxidation resistance and has a stabilizing effect on the Cr-N bonds. The onset of dissociation is shifted to temperatures >1000 °C where Al-Cr-N decomposes to form Cr via the intermediate step of Cr2N under the release of nitrogen. Polycrystalline Al0.68Cr0.32N films show precipitation hardening due to wurtzite AlN formation, however, causing reduced Cr-N bond stability. Suppressing precipitation by reduction of possible nucleation sites (i.e. single crystal films) or lower Al-content in the film results in a conserved hardness of 30 GPa up to 1000 °C.