Recent findings about the migration, life history, and habitat use of several species of diadromous fishes that have been examined using analysis of otolith strontium (Sr) and calcium (Ca) concentrations are overviewed. The Sr concentration or Sr:Ca ratios along life history transects in the otoliths of these fishes fluctuated greatly in accordance with their migration patterns between freshwater and marine habitats, enabling each migration pattern to be distinguished. In Anguilla, the mean Sr:Ca ratios indicated that there were eels with several general categories of migratory history, that included sea eels that never entered freshwater and estuarine eels that had entered freshwater for brief periods, but returned to the estuary or bay. The life cycle of Anguilla that migrate between the ocean and freshwater has been generally accepted without question, while the discovery of sea and estuarine eels has greatly changed the understanding of Anguilla life history.  Although Gasterosteus has been classified into anadromous and freshwater resident forms based on its life-style and morphology, continuous high Sr:Ca ratios from the core to the edge of the otolith were found in both forms. Therefore, besides the two representative migration forms of Gasterosteus, some individuals appear to spend their entire lives in brackish and/or seawater as estuarine residents. Salangichthys and Cottus showed a similar diversity of movements between freshwater and seawater as was seen in Anguilla and Gasterosteus. These findings suggest that many diadromous fishes may have flexible migration strategies and an ability to adapt to various habitats and salinities.