X-ray microanalysis of materials using energy-dispersive spectrometry (EDS) has made the greatest impact in studies of compositional changes at atomic-level interfaces. The small physical dimensions of the silicon detector make EDS the X-ray analyzer of choice for analytical transmission electron microscopy (AEM). X-ray analysis of thin foils in the AEM has contributed to our understanding of elemental segregation to interphase interfaces and grain boundaries, as well as other planar defects. Measurement of atomic diffusion on a small scale close to interphase interfaces has permitted determination of substitutional atomic diffusivities several orders of magnitude smaller than previously possible and has also led to the determination of low-temperature equilibrium phase diagrams through the measurement of local interface compositions. Elemental segregation to grain boundaries is responsible for such deleterious behavior as temper embrittlement, stress-corrosion cracking, and other forms of intergranular failure. On the other hand, segregation can bring about improvement in behavior: sintering aids in ceramics and de-embrittlement of intermetallics. EDS in the AEM has been responsible for quantitative analysis of all aspects of the segregation process and, more recently, in combination with electron energy-loss spectrometry (EELS) has given insight into why boundary segregation results in such significant macroscopic changes in properties.