This paper describes procedures by which two types of measures of Pedagogical Content Knowledge (PCK) were developed and validated: (a) PCK Survey and (b) PCK Rubric. Given the topic-specificity of PCK, the measures centered on photosynthesis as taught in high school classrooms. The measures were conceptually grounded in the pentagon model of PCK and designed to measure indispensable PCK that can be applied to any teacher, in any teaching context, for the given topic. Because of the exploratory nature of the study, the measures focus on two key components of PCK: (a) knowledge of students' understanding in science and (b) knowledge of instructional strategies and representations. Both measures have established acceptable levels of reliability as determined by internal consistency and inter-rater agreement. Evidence related to content validity was gathered through expert consultations, while evidence related to construct validity was collected through analysis of think-aloud interviews and factor analyses. Issues and challenges emerging from the course of the measure development, administration, and validation are discussed with strategies for confronting them. Directions for future research are proposed in three areas: (a) relationships between PCK and teaching experiences, (b) differences in PCK between science teachers and scientists, and (c) relationships between PCK and student learning.