In this paper, we report on 2 studies developing, testing, and using an observation tool for measuring primary literacy instruction, the Developing Language and Literacy Teaching (DLLT) rubrics. In Study 1 (an instrumentation study), we show that the DLLT has a high level of internal consistency, that there are high levels of inter-rater reliability when the tool is used by in-school coaches, that the items order consistent with a hypothesized developmental trajectory, and that the DLLT can distinguish between novice and more experienced teachers at one time point. In Study 2 (a 3-year longitudinal study), we show that the DLLT is sensitive to growth in teaching practice, that this growth is related to exposure to one-on-one coaching, and that teacher practice as measured by the DLLT is related to teachers' value added to student achievement year by year.