Both California state law and the federal No Child Left Behind Act (NCLB) require that all schools assess the English language proficiency of newly enrolled students who speak a language other than English at home and, annually, all English learners (ELs) already enrolled. California meets this requirement by administering the California English Language Development Test, or CELDT. The CELDT has three primary purposes: to identify students who are ELs, determine their English proficiency level, and assess their progress in acquiring listening, speaking, reading, and writing skills in English through time. We examine data on the validity and reliability of the CELDT to determine if it is an appropriate tool for carrying out these purposes. We conclude that the CELDT is likely a sufficiently valid and reliable tool for making judgments about groups of students but not for making crucial educational decisions about "individual" students.