In the current debate on pedagogical content knowledge (PCK), the term is used to refer to the context-specific knowledge that teachers activate when reflecting on practice. Against the background of this debate, we conducted an empirical study and sought to answer the question of which knowledge experts and novices activated in assessing a videotaped lesson in relation to its effectiveness for learning. Our assumption was that the participants activate their PCK as a blending of content knowledge (CK) and pedagogical knowledge (PK) as suggested by Shulman's amalgam thesis. The participants (9 experts and 9 novices) were shown a lesson on optics, in which the law of refraction (Snell's law) was being studied. In a subsequent interview, the participants were asked to analyze the observed lesson. A qualitative and quantitative evaluation of the interviews showed that experts activated both CK and PK intensively and in this respect they differed significantly from novices. Further analysis of the expert statements also proved that they do not activate their CK and PK in isolation, but instead combine both kinds of knowledge together, in line with Shulman's amalgam thesis. # 2017 Wiley Periodicals, Inc. J Res Sci Teach 55: 2018 Keywords: pedagogical content knowledge; science teacher education; professional development; teacher cognition Discussions on the nature of pedagogical content knowledge (PCK) have been circulating for around three decades. The origin of these discussions can be traced back to the works of Shulman (1986Shulman ( , 1987 and his concern that the efforts being made at the time to prepare pre-service teachers were largely focused on imparting general pedagogical skills while at the same time questions regarding the content to be taught in lessons were being neglected. Therefore, he called for teacher education to be more strongly focused on content in the future and felt that content and pedagogy should be viewed as "part of one indistinguishable body" (1986, p. 6).This demand was linked to a classification of teacher knowledge, in which Shulman differentiated between seven components of knowledge: content knowledge, general pedagogical knowledge, curriculum knowledge, pedagogical content knowledge, knowledge of learners and their characteristics, knowledge of educational contexts, and knowledge of educational ends, purposes, and values, and their philosophical and historical grounds (1987). Within this structure for classifying knowledge, however, Shulman placed particular emphasis on PCK. "Among those categories, pedagogical content knowledge is of special interest because it identifies the distinctive bodies of knowledge for teaching. It represents the blending of content and pedagogy into an understanding of how particular topics, problems, or issues are organized, represented and adapted to the diverse interests and abilities of learners, and presented for instruction." (1987, p. 8) This definition of PCK was linked to Shulman's "amalgam thesis": PCK is "that specific amalga...
