Research into teacher learning and practice over the last three decades shows that the teachers of a specific subject need to possess knowledge that is different from the knowledge of other content experts. Yet this specialized version of content knowledge that teachers need to plan instruction, respond to student ideas, and assess student understanding in real time is a theoretically elusive construct. It is crucial for the fields of precollege teacher preparation, teacher professional education, and postsecondary faculty professional development to (a) clarify the construct that underlies this specialized content knowledge, (b) operationalize it in some domain, (c) measure it in both static contexts and as it is enacted in the classroom, and (d) correlate its presence with "richness" of classroom instruction and its effect on student learning. This paper documents a piece of a multiyear, multi-institutional effort to investigate points (a)-(d) in the domain of energy in the first high school physics course. In particular, we describe the framework that we developed to clarify content knowledge for teaching in the context of high school energy learning. We then outline the process through which we developed, tested, and refined a "paper-and-pencil" assessment administered on a computer and discuss the substantive and psychometric features of several items based on a field test of the final form of the assessment. We choose to discuss these items for a dual purpose: to illustrate the application of our general framework and to present performance findings from a sample of 362 practicing high school teachers of physics.