Atom probe tomography (APT) has significantly contributed to our understanding and development of structural materials through the detailed analysis of solute behavior, cluster formation, precipitate evolution, and interfacial and grain boundary chemistry. Whether one is concerned with light alloys, Ni-based superalloys, or steels, the design objectives are similar: developing alloys with optimum properties (strength, toughness, ductility, fatigue resistance, creep strength) through controlled precipitation, grain structure, solute state, and combination of phases. Performance in service, through microstructural stability and resistance to degradation, is also a major design criterion for the development of novel materials.