Despite the promise of performance assessments in contributing to positive changes in instruction and assessment, more studies are needed that examine possible language biases that arise when assessing complex content understanding. The focus of this study was to investigate the impact of students language background on complex performance assessments. An additional aim was to explore accommodation strategies to assess content understanding of English language learners (ELLs) by manipulating both the reception and response modes of cognitively complex performance assessments. Accommodations to the reception mode involved linguistic modifications to a history explanation task based. Accommodations to the response mode involved the use of knowledge maps designed to graphically depict a student s knowledge and are considered less linguistically demanding. The results indicate that scores on complex performance assessments underestimate ELLs content understanding when linguistic accommodations are not provided. In addition, language background was found to be useful in determining the most appropriate linguistic accommodation for ELLs.