Writing in higher education is more than a basic skill that undergraduates master prior to coming to college or in their first year writing courses. Instead, students learn to write throughout their undergraduate experience and beyond. What fosters this writing development? What knowledge, skills, and capabilities develop as students progress through their undergraduate years and into graduate school of the workplace? This dissertation addresses the processes and outcomes of writing development drawing on data gathered in the Stanford Study of Writing. The study addresses the processes and products of writing development through an analysis of 150 interviews with 40 students conducted over a five year period, and a criterion referenced scoring of 200 pieces of student writing across the five years of the study. Analysis of this data indicates that students develop through social interactions that take place at two levels: First, through interactions with the college curriculum itself; secondly, and more importantly, students grow through dialogic interactions, that is, writing focused conversations (written or spoken) that are ongoing, invite active response, and are addressed to individual writers' thinking and ideas. The actual mechanism of development is fine tuning in which writers adjust, synchronize, and coordinate their behavior with readers and teachers, as well as disciplinary and professional communities of practice. The findings of this study show that, as students move through the undergraduate curriculum and pursue disciplinary specializations, students show statistically significant growth in a number of important knowledge domains: rhetorical knowledge, and writing processes knowledge, genre knowledge, and domain specific content knowledge, including disciplinary methods of critical thinking. The central findings of this study are that writing develops across multiple dimensions in a non-linear fashion through dialogic interaction and fine tuning in the direction of increased, specialized, social participation. Thus, writing development is fostered by a particular form of specialized social interaction in a social direction.