Polymer microsystem packaging materials have been characterized and failure analysis methods have been developed with the aim of gaining higher microsystem reliability. The importance of this work stems from the fact that microsystem sensors due to small size are very sensitive to the often very aggressive surroundings. Focus is on how the adhesion of protective polymer adhesives and coatings can be characterized theoretically and practically and optimized regarding intrinsic properties, the surroundings and their mutual influences. The main conclusion is that the mutual influences make a system design approach to development of reliable microsystem packaging mandatory. Diffusion of water is identified as the most important parameter or physical mechanism lowering microsystem reliability due to corrosion, delamination etc. This topic is therefore treated thoroughly by mathematical modeling / practical calculations to find diffusivities and methods are given by which water can be kept away from critical areas in microsystems.