The dominant narrative for assessment design seems to reflect a strong, albeit largely implicit undercurrent of purpose purism, which idealizes the principle that assessment design should be driven by a single assessment purpose. With a particular focus on achievement assessments, the present article questions the tenability of purpose purism, explaining how critical decisions--concerning whether to assess, how to specify an assessment construct, and many other design characteristics--require the coordination of multiple perspectives on assessment purposes. It argues the case for purpose pluralism--which idealizes the principle that assessment design should be driven by a multiplicity of assessment purposes simultaneously--not as an occasional, unavoidable concession, but as an organizing principle. The point of explicitly distinguishing between perspectives is to help assessment designers to establish a full complement of design requirements, representing a full range of stakeholder voices; as well as to manage more effectively the trade-offs and compromises that inevitably arise.