In this article, a classroom teacher discusses ambiguities in mathematics vocabulary and strategies for ELL students in building understanding. The authors note that mathematics vocabulary may be more difficult to learn than other academic vocabulary for several reasons: (1) definitions are filled with technical vocabulary, symbols, and diagrams; (2) many mathematics concepts can be represented in multiple ways; (3) many mathematics words have multiple meanings; (4) the overlap between mathematics vocabulary and everyday English is problematic; (5) homonyms and words that sound similar can be confusing; and (6) similarity to native language words can add more confusion.Clearly, vocabulary is an important issue in mathematics classrooms, especially for ELL students, and this article provides a selection of strategies for supporting students' development of mathematics vocabulary and examples of how to use them. Additional suggestions illustrate vocabulary support within an algebra unit, but could be adapted for other topics. Two tools are highlighted: (1) word walls--organized collections of words displayed in the classroom to support vocabulary development; and (2) graphic organizers--visual charts and representations designed to organize student learning. The authors conclude that, along with reading research literature, mathematics teachers should build their own understanding of the challenges that their ELL students face.