Educators of young children, including speech-language pathologists, are often uncertain as to how to effectively work with children from diverse backgrounds because they do not know enough about cultural and linguistic diversity and its impact on language development. The current study helps to address this gap by examining the validity of parent and teacher report in determining language proficiency and language ability in prekindergarten and kindergarten age children from Spanish-English bilingual backgrounds. Parents and teachers rated child language proficiency and ability in Spanish and English. Results indicate that teachers and parents were reliable informants on English language proficiency, but only the parents reliably rated children's Spanish proficiency. Both teacher and parent report were significantly correlated to child language ability. Teachers' ratings of ability correlated with morphosyntax performance while parents' ratings correlated with their child's broad language performance. For clinical and educational decision-making, we emphasize the importance of understanding bilingual children's language use across languages and contexts by incorporating both parent and teacher observations. (Contains 8 tables.)