The Lifespan Development of Writing presents the results of a four-year project to synthesize the research on writing development at different ages from multiple, cross-disciplinary perspectives, including psychological, linguistic, sociocultural, and curricular. Although writing begins early in life and can develop well into adulthood, we know too little about how writing develops before, during, and after schooling, as well as too little about how an individual's writing experiences relate to one another developmentally across the lifespan. There is currently no adequate accepted theory of writing development that can inform the design of school curricula and motivate appropriate assessment practices across the years of formal education. The Lifespan Development of Writing is a first step toward understanding how people develop as writers over their lifetimes and presents the results of a four-year project to synthesize the research on writing development. First collectively offering the joint statement "Toward an Understanding of Writing Development across the Lifespan," the authors then focus individually on specific periods of writing development, including early childhood, adolescence, and working adulthood, looked at from different angles. They conclude with a summative understanding of trajectories of writing development and implications for further research, teaching, and policy, including the assertion that writing research "can raise our curricular vision beyond the easily measurable to recognize that writing development is far more than the accretion of easy testable skills, and that successful writing development cannot be defined as movement toward a standard."