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Pedagogical Content Knowledge (PCK) is the blending of content knowledge and pedagogical knowledge for the purpose of teaching in ways that are comprehensible for learners. Although PCK was initially termed by Shulman in 1986, it has never been operationally defined. This study was based on two primary assumptions: (a) Pedagogical Content Knowledge can be predicted by a set of operational variables, and (b) the conceptualization that PCK develops on a continuum from one end to another, from immature to mature forms of PCK. The purpose of the current investigation was to: (a) test a proposed operational definition of PCK, and (b) to examine how the PCK of experienced teachers differ in the teaching of their stronger and weaker units of instruction. The study was a descriptive investigation based on the behavior analytic theoretical framework. The investigation was organized into three phases. Phases 1 and 2 included a focus group and individual interviews to (a) gain initial understanding on the teachers ’ learning history relative to PCK; (b) identify possible contingencies the teachers ’ behavior is operating under, that cannot be found by observing the immediate teaching context; and (c) discover variables related to PCK that could be observed durin