The purpose of this report is to propose a theory of action for the development of an alternate English language proficiency assessment (AELPA) system to support the integrated instruction and assessment for English learners with significant cognitive disabilities (ELSCDs). This theory of action examines the purposes of an ELP assessment system based on the Every Student Succeeds Act (ESSA) regulations requiring the assessment of all ELSCDs. This report seeks to inform assessment considerations for the development of an AELPA system for state education agencies (SEAs) and test developers. This theory of action identifies assumptions, purposes, and goals for assessment development and includes a logic model to represent system claims. The report ends with a discussion of unintended consequences and ways to mitigate them, as well as future research recommendations.