Expert science teachers possess a special blend of science content knowledge and pedagogical knowledge for teaching particular science topics to particular groups of students that is built up over time and experience. This form of professional knowledge, termed pedagogical content knowledge (PCK) by Shulman (1987), is topic-specific, unique to each science teacher, and can only be gained through teaching practice. The academic construct of PCK is a recognition that teaching is not simply the transmission of concepts and skills from teacher to students but, rather, a complex and problematic activity that requires many and varied ‘on the spot’ decisions and responses to students’ ongoing learning needs. Much has been written about the nature of PCK since Shulman first introduced the concept in 1987, and its elusive characteristics have led to much debate.