Аннотация:A short overview is given of the contribution of transmission electron microscopy (TEM) to the structural characterization of phase transformations, nucleation, and growth of precipitates in iron and different steels. The development of understanding of mechanisms governing these transformations is illustrated by examples. In particular, attention is paid to the development of precipitates in pearlite, bainite, and martensite microstructures, nucleation and formation of precipitates of carbides, nitrides, and Guinier–Preston zones. The precipitation processes during tempering and phase cycling are illustrated.