Using a nationally representative dataset of young children in the United States (the Early Childhood Longitudinal Study--Birth Cohort), the purpose of this study was to test the associations between teachers' perceptions of preschoolers' (N = 3350) school readiness and actual academic readiness levels, as measured by math and reading assessments. The dimensions of readiness included social/emotional, behavioral, cognitive, and physical well-being. The findings suggest that teachers' perceptions of various aspects of readiness may matter differently for math and reading achievement as well as for certain racial/ethnic groups. Teachers' perceptions of all domains of readiness appear to be an important predictor of math achievement for Black children. Perceptions of behavior were negatively associated with academic readiness for Hispanic children, yet significantly and positively associated with math achievement for Black children. Teachers' perceptions of cognitive readiness were only positively associated with academic readiness for Black children. Training, education, and support for establishing close teacher-child relationships may maximize preschoolers' academic readiness by promoting social/emotional, behavioral, cognitive, and physical well-being.