This chapter discusses the affective domain agency, which can also be described as personal control, epistemic agency, personal and collective responsibility, or agentic learning, and is, in the context of writing, about having choice, freedom, autonomy, and ownership of writing ideas, writing processes, and even how you are taught. Agency is a vital force in increasing writers’ engagement and their writing performance. Theory, research, and educational practices associated with developing children’s agency in writing and its important role in developing confident and effective writers are explored within this chapter. This includes discussion of the typical behaviours witnessed in a rich variety of educational research and differences observed in the behaviour of pupil-writers who are provided with high and low levels of agency. Finally, the authors describe the relationship between developing apprentice writers’ agency and the 14 principles of effective writing teaching. They give examples of the kinds of instructional practice carried out by the most effective teachers of writing.