This article reviews key developmental theories that have been adopted by writing development researchers over the last fifty years. It describes how researchers have translated these theories into definitions of writing development capable of influencing curricular design and interpretations of student writing and explores the implications for assessment extended by each theory. This piece examines both the variations in assessment practices/emphases suggested by different developmental theories as well as the shared assumptions about growth to which they lead, arguing that these commonalities should guide efforts to foreground development in the assessment of writing.