This paper identifies four misconceptions about language proficiency which are currently impeding the implementation of effective bilingual education programs. These misconceptions involve: 1) the attribution of deficient language or cognitive skills on the basis of non-standard varieties of L1, and the consequent attempt to eradicate these stigmatized varieties; 2) the attribution of “English proficiency” to language minority students on the basis of adequate surface structure in “context-embedded” face-to-face communicative situations, with the result that low English academic performance in “context-reduced” communicative situations is attributed to deficient cognitive abilities; 3) the belief that L1 and L2 proficiencies are separate rather than manifestations of a common underlying proficiency; 4) the belief that a home-school language switch, or “linguistic mismatch”, is the major cause of language minority students’ academic failure.