Dual language immersion (DLI) as an alternative model to monolingual English-medium education has continued to expand rapidly over the past decade. As this model continues to grow across the United States, stakeholders look to research to demonstrate sustainability and scalability of dual language programs. Drawing on 224 schools, the statewide study reported in this article investigated whether students' growth in a non-English DLI partner language sustained into the secondary level, to what degree students met performance benchmarks for these partner languages, and whether targeted outcomes were attained during program expansion. The longitudinal analysis of assessment data shows that middle school students performed at Intermediate Mid to High levels on the American Council for the Teaching of Foreign Languages (ACTFL) proficiency scale in listening, speaking, reading, and writing, and that a segment of students raised their performance levels from 8th to 9th grade. Furthermore, a considerable portion of DLI students in French and Spanish dual language programs reached the Advanced level in 9th grade. Finally, 3rd- and 4th-grade data show that targeted performance outcomes held from one testing year to another, 4 years later, and after a substantial increase in enrollments. The article concludes with a discussion of curricular and policy implications.