Harrisonburg, Virginia, a community nestled in the fertile hills of the Shenandoah Valley, is emblematic of the demographic changes taking shape in the U.S. for some years now. The town's agricultural industry has attracted a large number of immigrant workers from Central America. In addition, Harrisonburg's refugee resettlement center has drawn many other international families to the area. "A Critical Mass: Creating Comprehensive Services for Dual Language Learners (DLL) in Harrisonburg" explores Harrisonburg City Public Schools' comprehensive system of instructional programs, social-emotional supports, and family engagement strategies targeted to meet the needs of its DLL students. This reports describes how Harrisonburg has implemented multiple initiatives that align with what the research says are best practices for DLLs, including creating Spanish-English dual immersion programs to help DLLs become bilingual and biliterate. The district also has differentiated services for DLL students who are new to the U.S. to help foster their integration and accelerate their English language acquisition. This report shares several lessons for other localities seeking to enhance and reform services for their DLLs that include: (1) Investing to expand DLL access to early childhood education programs; (2) Creating welcome centers, hiring liaisons, and building partnerships to increase family engagement in schools; (3) Differentiating programs to meet the diverse needs of DLLs; (4) Preparing all teachers to work effectively with DLLs; (5) Exploring strategies to increase collaboration among schools; and (6) Incorporating DLL needs at the outset of all policy formulation and reformulation.