In this study, we examined the relation of observed classroom practices to language and literacy achievement and the moderation of this relation for students from pre-K to sixth grade. A total of 136 studies (N = 107,882 participants) met the inclusion criteria, of which 108 studies were included for meta-analysis and the other 28 studies were narratively synthesized. The average zero-order (r = 0.12) and partial correlations (r[subscript p] = 0.04) were statistically significant but weak in magnitude. The relation was slightly weaker in upper than in lower grade levels, and stronger for observations capturing macro quality and instructional dimension than those capturing micro measurement and emotional or structural dimension, respectively. The relation did not vary by observation duration, frequency, adopted statistical approach, or type of covariates. Taken together with the narrative synthesis, the results highlight the complex nature of classroom observation and a need for more classroom research, particularly on higher grade levels. [This is the online version of an article published in "Review of Educational Research."]