This study describes changes in secondary mathematics teachers’ mathematical knowledge for teaching function through their engagement in a mathematics methods course teaching experiment. The participants 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. The course focused on function which supports work in the classroom; by focusing on one topic, teachers experience the sequencing of tasks and topics in ways that build a conceptual understanding, much in the way that they might design a curricular sequence in their own classroom.Furthermore, the course activities provided teachers with opportunities to refine and elaborate those initial understandings.