The aim of the present article is to highlight two examples of the unique role that transmission electron microscopy (TEM) can play to solve complex materials problems. The strength and fatigue resistance of cast Al alloys can be increased by adding copper and magnesium. TEM work revealed that a cylinder head alloy (Al-8Si-3Cu-0.3Mg) was effectively age-hardened by the formation of finely dispersed precipitates of ϑ" (CuAl2) and β" (Mg2Si) phases. The two precipitates were distinguished by the TEM. The geometry, the orientation, the shape and the size, etc., of the precipitates were completely characterized. A new type of precipitate containing Mn, Si, Cu and Al and finely dispersed in the matrix was discovered in this work. The second example is a TEM/STEM study of a grey iron. Alloying additions of, for example, Mo and Cu in the grey iron have a considerable effect in the strengthening of the alloy. It was found that Mo toughened the ferrite/Fe3C interface by forming a multi-layer structure on the surface of the cementite, and the ferrite matrix was strengthened by solution of Cu in the solid state and by a fine dispersion of coherent precipitates