In order to teach mathematics effectively, mathematics teachers need to have a sound mathematical knowledge, but what constitutes sound mathematical knowledge for teaching is subject to debate. This paper is an attempt to unpack what constitutes teacher knowledge of the concept of a function which is a unifying idea in the mathematics curriculum. The central components of the framework, which will be elaborated on in this paper, are: teachers' subject matter knowledge, teachers' pedagogical content knowledge, teachers' technological pedagogical knowledge, technological content knowledge, and technological pedagogical content knowledge in relation to the concept of a function. The framework is informed by Shulman's ("Educational Researcher" 15:4-14, 1986) "Types of Teachers Knowledge Framework," Ball, Bass & Hill 29:14-17, 20-22, 43-46 (2005)" Mathematical Knowledge for Teaching Framework," and Mishra & Koehler's ("Teachers College Record" 108:1017-1054, 2006) Technological Pedagogical Content Knowledge (TPACK) framework.