Professional learning communities (PLCs) provide an increasingly common structure for teachers' professional development. The effectiveness of PLCs depends on the content and quality of the participants' discourse. This dissertation was conducted to add to an understanding of the science content needed to prepare to teach science, and the discourse characteristics that create learning opportunities in teachers' PLCs. To this end, this study examined how middle school science teachers in three PLCs addressed science-for-teaching, and to what effect. Insight into discourse about content knowledge for teaching in PLCs has implications for the analysis, interpretation, and support of teachers' professional discourse, their collaborative learning, and consequently their improvement of practice. This dissertation looked closely at the hybrid space between teachers' knowledge of students, of teaching, and of science, and how this space was explored in the discourse among teachers, and between teachers and science experts. At the center of the study were observations of three 2-day PLC cycles in which participants worked together to improve the way they taught their curriculum. Two of the PLC cycles were supported, in part, by a science expert who helped the teachers explore the science they needed for teaching. The third PLC worked without such support. The following overarching questions were explored in the three articles of this dissertation: (1) What kind of science knowledge did teachers discuss in preparation for teaching? (2) How did the teachers talk about content knowledge for science teaching, and to what effect for their teaching practice? (3) How did collaborating teachers' discursive accountabilities provide opportunities for furthering the teachers' content knowledge for science teaching? The teachers' discourse during the 2-day collaboration cycles was analyzed and interpreted based on a sociocultural framework that included concepts from the practice-based theory of content knowledge for teaching developed by D. L. Ball, Thames, and Phelps (2008) and the Accountable Talk framework by Michaels, O'Connor, & Resnick (2008). The study's findings could provide justification for and ideas on how to provide targeted support for PLCs to make teachers' work on science knowledge more applicable to lesson planning, teaching, and student learning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]