In this study, we explored teachers' attention to and flexibility with referent units as well as how teachers' understanding of referent units is related to their performance on other fraction concepts and their professional background. By using data collected from 246 U.S. mathematics teachers in Grades 3-7 where fractions are taught, we found that teachers' attention to and flexibility with referent units were moderately related. Whereas some teachers' professional background variables could explain their flexibility with referent units, none of the variables was linked to their attention to referent units. Furthermore, both teachers' attention to and flexibility with referent units seemed to be associated with their performance on other fraction concepts. [For the complete proceedings, see ED629884.]