The aim of this research is to describe the development of the Science Teachers’ Pedagogical Discontentment Scale, an instrument that measures the discontentment that arises in teachers as they recognize a mismatch between their own pedagogical beliefs and goals and their actual classroom practices. From a conceptual change perspective, we explore the meaning of pedagogical discontentment and discuss its role in shaping teachers’ receptivity to messages of reform. We present an instrument that can be used to measure teachers’ pedagogical discontentment, an instrument that will allow science educators to better describe the affective states of teachers as they enter professional development experiences. The items for the initial instrument were derived from a series of interviews with practicing teachers; from these interviews, a group of 42 items were designed around a group of five subscales. The final instrument, revised after two rounds of field testing, includes 21 multiple-choice items clustered around six subscales (subscales derived from interviews with science teachers). The processes used to develop the items and to refine instrument are discussed. Uses for this instrument to inform professional development experiences are explored as well as implications.