This document discusses pedagogical content knowledge (PCK) for secondary school teachers. It outlines the history and development of PCK, starting with Shulman's identification of PCK as the third dimension of teacher knowledge in 1986. It then summarizes several models of PCK proposed by other researchers between 1988-2011 to define PCK and its components. The document aims to provide an overview of the taxonomy and evolution of PCK as an important factor in teacher quality and student academic achievement.