Continuous miniaturization of electronic devices increases the importance of the understanding and control of the physical and chemical compatibility between dissimilar materials in order to design and fabricate reliable products. Thermodynamic and kinetic modelling together with careful experimental work is of great help for understanding and controlling interdiffusion and chemical reactions in interconnection structures. The approach provides useful information on the stabilities of phases (microstructures), driving forces for chemical reactions and growth rates of reaction products occurring in interconnections or thin film structures during processing, testing and in long-term use of electronic devices. In this paper two examples of the approach outlined above are presented. Firstly, the case of TaC diffusion barrier between Cu and Si at the IC-interconnection level is discussed. Complex reactions occur in this system especially when oxygen is present as an impurity. Evaluated Si-Ta-Cu, Ta-C-O and Si-Ta-C ternary stable as well as metastable phase diagrams are used together with the calculated activity diagrams and detailed transmission electron microscopy (TEM) results to rationalize the formation of the observed microstructure after extensive reactions. Secondly, reactions occurring between the Ni(P)/Au printed wiring board (PWB) surface finish and copper containing lead-free solders at the board level are presented. To understand the formation (Cu,Ni)6Sn5 instead of the expected Ni3Sn4 during soldering the ternary Sn-Cu-Ni phase diagram is used. The influence of the subsequent solid state annealing and the related mass-supply effects on the stability of the microstructure are also discussed.