ABSTRACTIn this paper the results of experiments are summarized in which the internal stress (i.e. force per unit width) of thin films was measured in situ under UHV conditions with a bending beam apparatus. It is demonstrated that characteristic types of stress vs. thickness curves can be correlated with different growth modes (i.e. columnar grain growth and island growth) caused by differences in the adatom mobility of the deposited material. With a selection of thin film systems it is then shown that stress measurements can be used to study the effect of gas incorporation, of gas diffusion from the substrate, of reaction with the substrate and of the substrate temperature on the growth and structure of a thin film. Finally it will be demonstrated that stress measurements can even be used to study gas adsorption on vapor deposited films.