Standards for mathematics teaching require teachers to employ teaching practices that promote justification of mathematical ideas. This expected teaching practice is situated in substantial research on students' and teachers' difficulties with justifying mathematical ideas. This study shows different ways elementary school students in grades three through five may justify mathematical conjectures about pattern-finding activities. It also shows that even when teachers are capable of justifying particular tasks, enactment of such tasks in ways that encourage students to go beyond example-based justifications may be problematic. Video and audiotapes of class activities, students' written work, and curriculum materials were sources of data.