This thesis describes advanced experimental research on the physical and chemical properties of bilayer and multilayer ultra-thin films. Atomic scale processes that occur at the interfaces of thin film stacks are known to dramatically influence their functional properties. This includes layer roughness and intermixing occurring during the film deposition process, as well as interdiffusion of layer materials and interlayer compound formation during usage of the thin-films, for example, exposure of an optically reflective multilayer stack to a high-power light source and electromigration due to current flow in integrated circuit devices. Currently, there is a critical demand for a broadly applicable material-selection-guide to design atomically sharp and stable layer stacks for various thinfilm devices. This thesis has given an important onset for that. It is focused on understanding the surface and interface diffusion mechanisms involved during: (i) near room temperature layer growth, and (ii) low temperature annealing of transition metal (TM) and Si based layered systems. The thesis presents a novel scaling law for the effective interface width between two layers, which can be used for the selection of suitable layer materials to achieve well-controlled interfaces in a thin-film stack.