The microstructural features that are associated with metals exhibiting the Bauschinger effect are reviewed. The Bauschinger effect is interpreted in terms of the polarization of work hardening due to internal stresses built up by accumulation of dislocations at obstacles. Methods of measuring these internal stresses are discussed and experimental results compared with the results of Bauschinger tests. It is concluded that a unique structural interpretation of the magnitude of the directional internal stresses is not yet possible, and that detailed structural data of this kind is required if further understanding of the mechanism of the Bauschinger effect is to be achieved.