The chemistry of solid–solid interfaces in metals is reviewed in relation to its characterization, theory, and relevance to materials science. The current techniques for measuring interface segregation, AES, STEM with x-ray analysis, and mass-resolved field desorption microscopy, are outlined and their advantages discussed. The expression of quantitative measurements from these techniques is unified. The behavior of segregation is discussed for the experimental variables—localization, crystal orientation, chemical system, time, and temperature. The theories to describe segregation in binary and ternary systems are then presented together with the analysis of the kinetics of segregation. The importance of these different aspects arises in a number of segregation sensitive materials properties. These will be described together with remedial procedures that result from the characterizations.