The purpose of the research presented in this paper has been to characterize flexibility in lean mixed model assembly lines through exploring mechanisms used to achieve flexibility. The study combines a literature review and a multiple case study in two manufacturing companies. Scenarios of volume, mix and operation flexibility, as well as flexibility to introduce or remove products were studied. The results showed that to achieve flexibility related to these scenarios other kinds of flexibilities were used. Common mechanisms to achieve flexibility have been found in the two cases. A characterization of mixed model assembly lines regarding flexibility will be discussed.