The study on defect and diffusion is an important area due to its present status in technology and so this book has its relevance still now. From the theoretical point of view Lattice Statics and Atomistic Continual Models in chapter one have produced, with all their approximations, some interesting and quite consistent results for relaxations due to a vacancy, its formation energies and divacancy binding energy in some FCC metals. Pseudopotential approach of chapter two produces general formulations for the calculation of defect parameters in case of vacancy in FCC metals for self and impurity diffusion. In the third chapter the experimental measurement of impurity diffusion parameters in Pb-Sn system is shown. The results are consistent with earlier experiments and theoretical values by pseudopotential method. To understand the kinetic behaviour of interfacial reactions in Ag-Sn thin film couples the contact and composite resistance measurements are done which indicate that interfacial reactions start at room temperature. X-ray diffraction indicates the growth of ?-Ag3Sn phase immediately after deposition and TEM and SEM studies confirm its presence.