California currently faces an opportunity to develop an effective data system that can assist in improving the state's future understanding of the educational progress of English learners (ELs). This policy brief outlines the needs for good data on ELs and makes recommendations for creating an effective longitudinal data system for ELs. It provides general advice on how to enhance the California Longitudinal Pupil Achievement Data System (CALPADS), which tracks individual students over time, and the California Longitudinal Teacher Integrated Data Education System (CALTIDES), which tracks individual teachers over time. Ultimately, legislators, practitioners, and researchers can all benefit from the creation of an effective longitudinal data system that better assesses the effectiveness of various programs serving the state's EL population.