This article report on the development and use of an analytical framework intended to map the language demands encountered by English learners as they engage in science performance assessments. Grounded in functional and interactional views of language and language use, the authors--two science education researchers and a language scholar--developed the framework via an inductive, iterative, and systematic review of written assessment materials associated with three fifth grade science performance tasks. The resulting Science Assessment Language Demands (SALD) framework is organized around three dimensions: participant structures, communicative modes, and written texts and genres that students are called upon to read and produce. The authors used textual analysis to conduct an expert review of the written documents associated with the three assessment tasks. The results indicate that the framework can be used to document a wide range of functional and interactional language demands involved in science performance assessments. The demands revealed by the SALD framework highlight both potential challenges facing English learners during science performance assessments as well as opportunities afforded by such assessments for demonstrating their knowledge and skills and further developing language proficiency. A major implication of the study is the potential use of the framework to evaluate the language demands and opportunities of science assessments used in classrooms with English learners. (Contains 7 tables, 2 figures and 4 notes.)