This study describes teacher learning in a teaching experiment consisting of a content-focused methods course involving the mathematical knowledge for teaching function. Prospective and practicing teachers in the course showed growth in their ability to define function, to provide examples of functions and link them to the definition, in the connections they could make between function representations, and to consider the role of definition in mathematics and the K-12 classroom. Written assessments, interview data, and class discourse analyses illustrate how the course supported the development of mathematical knowledge that built on individual teachers' prior knowledge as well as the development of a stronger collective understanding of function.