To explore the relationship between teachers' proficiency in Spanish and their pupils' achievement, a test designed to measure the teachers' proficiency in Spanish was developed as part of a battery of tests for teachers in bilingual education programs -the CE RAS Teacher Tests for Spanish/English Bilingual Education. [Center for Educational Research at Stanford University -in addition to the Spanish proficiency test, the test battery includes tests on methods of teaching English and Spanish as a second language, methods of teaching reading in Spanish, and knowledge of southwest Spanish.]The report which follows summarizes efforts to establish the reli ability and validity of the Spanish Proficiency Test. The relationship of pupils' performances on standardized and criterion-referenced tests to their teachers' proficiency in Spanish was explored. Based on two independent studies, it appears that the relationship between pupils' achievement in Spanish and English and their teachers' Spanish proficiency is a strong one.In outlining recommendations for teacher training and the certifi cation of bilingual teachers, private and state agencies have suggested that potential teachers demonstrate their proficiency in the target language (see, for example, California State Department of Educa tion, 1974; Center for Applied Linguistics, 1974). The belief that teachers cannot teach what they do not know makes common sense. However, few attempts have been made to validate this view. This report will outline a systematic effort to establish the relationship between teachers' proficiency in the second language -Spanish in this case -and their pupils' achievement.