The purpose of this paper is to describe the vexing issues that arise as researchers try to define and assess the knowledge teachers need to teach reading and writing effectively in the primary grades. Over the last several years, researchers have developed and tested an assessment system of teacher knowledge under a grant entitled, The Primary Grade Reading and Writing Teacher Knowledge Project (Reutzel et al., 2005). The research team developed and tested two subscales, a traditional multiple-choice test that measures the inert knowledge teachers possess about reading and writing instruction and an accompanying classroom observation instrument that measures the enacted knowledge teachers use to teach reading and writing to young students. This paper reports on five conceptually and methodologically vexing issues that arose as they developed and tested the assessment system. Issues revolved around the specific knowledge to measure, the evidence needed by various stakeholders as convincing evidence of that knowledge, the potential concerns related to the use of measures of teachers’ knowledge, queries about the best way to measure that knowledge, special problems the use of classroom observations present for measuring teacher knowledge, and the predictive validity of such knowledge measures.