The aim of this work is to show new insights into thin film stress development for different temperatures by in-situ X-ray diffraction. In the first part, the elastic, anisotropic stress-temperature (S-T) evolutions of epitaxial GaN on gamma-LiAlO2 were studied between room temperature (RT) and 600°C. The obtained stresses show an anisotropic behaviour. So, it was first time possible to measure anisotropic thermal and intrinsic stresses in epitaxial GaN by diffraction. In the second part, a new experimental technique is introduced (combination-method). It allows a quantification of X-ray elastic constants (XEC) of thin films on single crystal substrates. The approach is demonstrated at RT for Al- and Cu films. The advantage of the technique is to determine XEC without using any diffractometer attachments. Extending the technique of X-ray substrate curvature (XSC), a heating chamber (HC) was developed, enabling measurements at elevated temperatures. Using this HC, first measurements of temperature dependent XSC were done. The functionality is shown on Al-, TiN- and Cr films between RT and 400°C. Further, size and temperature dependencies of thermal strains of textured Al films were studied by diffraction between -100°C and 350°C. The experiments were performed on 50 to 2000nm thick Al films on Si. The obtained S-T evolutions show size effects and are discussed taking micro-structural information via SEM- TEM- and AFM into account.