Examples of interfaces in metal matrix composites (MMC) are presented in this paper; in particular, Al alloys reinforced with B4C or SiC particles; pure Al reinforced with melt-spun intermetallic ribbons (Ni3Al); Mg alloys reinforced with C long fibers; Ti alloys reinforced with double coated (C/TiB2) SiC fibers. Kinetic studies of artificial aging on Al 6061/SiCp and Al 6061/SiCw composites are also presented. Several analytical techniques have been used for these studies: a good knowledge of the interfaces in MMC is fundamental for the design of these materials.