The purpose of this study was to examine the validity of teacher evaluation scores that are derived from an observation tool, adapted from Danielson's Framework for Teaching, designed to assess 22 teaching components from four teaching domains. The study analyzed principals' observations of 713 elementary, middle, and high school teachers in Washoe County School District (Reno, NV). The findings support the use of a single, summative score to evaluate teachers, one that is derived by totaling or averaging all 22 ratings. The findings do not support using domain- or component-level scores to evaluate teachers' skills, because there was little evidence that these scores measure distinct aspects of teaching. The information that the total score provides predicts the learning of teachers' students. While the relationship is moderate, it is evidence to support interpreting the observation score as an indicator of teachers' effectiveness in promoting learning. The following are appended: (1) Supporting tables; and (2) Confirmatory factor analyses: Methods and findings. Contains notes.