The purpose of this study was to rethink the conceptualization of pedagogical content knowledge based on our descriptive research findings and to show how this new conceptualization helps us to understand teachers as professionals. This study was a multiple case study grounded in a social constructivist framework. Data were collected from multiple sources and analysed using three approaches: (a) constant comparative method, (b) enumerative approach, and (c) in-depth analysis of explicit PCK. The results indicated that (a) PCK was developed through reflection-in-action and reflection-on-action within given instructional contexts, (b) teacher efficacy emerged as an affective affiliate of PCK, (c) students had an important impact on PCK development, (d) students' misconceptions played a significant role in shaping PCK, and (e) PCK was idiosyncratic in some aspects of its enactment. Discussion centres on how these five aspects are related to teacher professionalism.