This study illustrates why preschool children who are dual language learners (DLLs) are not a homogeneous group. An empirically developed model of preschool DLL subgroups, based on latent class analysis, was presented. The model reflects three separate subgroups of DLL children present in many classrooms where DLL children are served: Bilinguals, Emergent Bilinguals, and Heritage Language Speakers. Differences in standard scores based on teacher ratings of the developmental progress of preschool children were detected between the DLL subgroups and, relative to non-DLLs, at the beginning and throughout the preschool year. Differences were also found among classrooms having varying concentrations of DLLs. These findings highlight the importance of classroom composition to the development and learning of all children and suggest that policy makers and administrators should strive for classrooms that represent balanced diversity considering DLL status, race, ethnicity, economic status, and children with special needs. (author abstract)