While contemplating a new online assessment framework for global corporations in Turkey, developers faced the dilemma of how to create a modern English for Specific Purpose (ESP) writing proficiency assessment. The result was the Communicative English Proficiency Assessment (CEPA)® Written Communication Assessment™, a computer-based, criterion-referenced proficiency test. In its creation, and with the consideration of ESP tests’ need to first analyze the specific target language use context and situation (Douglas, 2000), developers utilized 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 two of ECD’s five stages, domain analysis and domain modeling, are documented in this study along with their results and a discussion of challenges encountered. As the praxis of ESP assessment development theory is not well-established (Knoch & Macqueen, 2020), this study also attempts to shed light on how needs analyses might be conducted. And in keeping with the notion that assessment as argument is a cornerstone of test validation (Kane, 2006, 2012), this study reports on the use of ECD, conceiving of assessment practices as evidentiary arguments, to inform the design and development an ESP writing proficiency assessment.