This cross-sectional study used assessments developed by the Center for Applied Linguistics to examine the oral proficiency of 218 K-8 English-proficient students in 4 Spanish immersion programs. Following a comprehensive review of assessment results for English-proficient immersion learners, the article reports findings from statistical analyses. Ratings of student proficiency were significantly higher between Kindergarten and Grade 2 and between Grades 2 and 5; however, no significant differences were found between Grades 5 and 8, lending empirical support to the plateau effect identified in earlier immersion studies. Furthermore, positive moderate to strong correlations were found between teacher ratings and ratings assigned by trained assessment administrators. The article discusses implications for assessment tools and practices, immersion program design, and pedagogy.