Written communication is considered one of the most critical competencies for academic and career success, as evident in surveys of stakeholders from higher education and the workforce. Emphasis on writing skills suggests the need for next-generation assessments of writing proficiency to inform curricular and instructional improvement. This article presents a comprehensive review of definitions of writing proficiency from key higher education and workforce frameworks; the strengths and weaknesses of existing assessments; and challenges related to designing, implementing, and interpreting such assessments. Consistent with extant frameworks, we propose an operational definition including 4 strands of skills: (a) social and rhetorical knowledge, (b) domain knowledge and conceptual strategies, (c) language use and conventions, and (d) the writing process. Measuring these aspects of writing requires multiple assessment formats (including selected-response [SR] and constructed-response [CR] tasks) to balance construct coverage and test reliability. Next-generation assessments should balance authenticity (e.g., realistic writing tasks) and psychometric quality (e.g., desirable measurement properties), while providing institutions and faculty with actionable data. The review and operational definition presented here should serve as an important resource for institutions that seek to either adopt or design an assessment of students' writing proficiency.