This study examined secondary mathematics teachers' content knowledge for teaching the concept of function. Content knowledge, for the purpose of this study, includes the common content knowledge and specialized content knowledge domains of Ball and her colleagues' Mathematical Knowledge for Teaching (MKT) framework. Data were collected through a 10-item questionnaire from 42 teachers practicing at fifteen different industrial vocational high schools in Ankara, Turkey. Findings revealed that most of the teachers placed a strong emphasis on the set correspondence and algebraic representations; found the uniqueness condition of functions very important for students to understand; and had a wide repertoire of examples of functions which constitute the uniqueness condition. The teachers' choice of examples mostly included giving set correspondence relations and checking whether they define a function. Additionally, a strong focus on the computational aspects associated with functions was evident. The teachers' understanding of the function concept might be influenced by the curricular materials, contextual factors, and high-stakes testing. The methodological and practical implications of these findings are discussed, and directions for future studies are offered.