CEPA® Written Communication Assessment™ is a computer-based, criterion-referenced, and proficiency-oriented test designed to measure non-native speakers' workplace English proficiency in writing. In its creation, developers used the evidence-centered design (ECD) approach (Mislevy & Haertel, 2006) to define and operationalize the construct of writing in the context of modern workplace communication. The first phase of ECD, Domain Analysis, yielded a comprehensive definition of 21st-century workplace written communication, revisions to CEFR language-proficiency descriptors, and an adaptation of the GRASPS framework (Wiggins & McTighe, 2008). These discoveries served the Domain Modeling phase by defining proficiency-oriented performance task design patterns; after which, a Conceptual Assessment Framework could be constructed. Due to the essential expansive detailing of each phase, this paper focuses only on the first three 'assessment conceptualization' phases of ECD. That is, the discussion of the last two 'assessment operationalization' phases, the Assessment Implementation and Delivery, is beyond the scope of the current paper. In keeping with the notion that assessment as argument is a cornerstone of test validation (Kane, 2006;, this study reports on the use of ECD's first three phases, conceiving of assessment practices as evidentiary arguments, to inform the design and development of the CEPA Written Communication Assessment.