In this dissertation, the researcher examined teachers' understandings and attitudes about the teaching of argument and how professional development (PD) affected those understandings and attitudes. The teaching of argument is important because argument promotes critical thinking skills, authentic learning opportunities, increased conceptual development, and meaningful discussions about topics that are important to students. The researcher gathered data about 14 teachers’ understandings and attitudes and looked at them as a qualitative case study bounded by their common experience in a PD opportunity during the summer with return visits to PD in the fall and the following spring. The findings include: (1) teachers had little to no previous experience with argument in their own learning or teaching, (2) the teaching of a new strategy, like argument, requires ongoing, systematic PD, and (3) after PD the teachers developed more complex, nuanced understanding of and value for the teaching of argument. These findings were significant in that preservice teacher programs, district PD opportunities, and teacher-led PD learning communities must strategically and intentionally address argument for its power in the 21st century classroom.