Erin has been teaching at Centennial High School for five years. Always an excellent mathematics student, she is nonetheless surprised at the amount of mathematics she has learned since she began teaching. She recently assigned what she thought was a fairly straightforward proportional reasoning task, only to be challenged to understand the mathematics in her students' solutions. Her journey to understanding the subtleties of potential solutions and what student approaches suggest for teaching provides a framework for characterizing content knowledge for teaching mathematics. This article illustrates the research done by the Mathematics Content Collaboration Communities (MC[superscript 3]) project, a partnership among faculty from Eastern Washington University, the University of Washington, Evergreen State College, and the University of Michigan. The MC[superscript 3] project defines, describes, and characterizes the mathematics knowledge needed for teaching high school mathematics. The experiences attributed to Erin are a composite of those of partner teachers in the project and are representative of the authors' findings. (Contains 9 figures.)