This article addresses the issue of language-related construct-irrelevant variance on content area tests from the perspective of systemic functional linguistics. We propose that the construct relevance of language used in content area assessments, and consequent claims of construct-irrelevant variance and bias, should be determined according to the degree of correspondence between language use in the assessment and language use in the educational contexts in which the content is learned and used. This can be accomplished by matching the linguistic features of an assessment and the linguistic features of the domain in which the assessment is measuring achievement. This represents a departure from previous work on the assessment of English language learners' content knowledge that has assumed complex linguistic features are a source of construct irrelevant variance by virtue of their complexity.