Individual differences factors (IDs) play significant roles in second language (L2) learning. Compared to the general field of SLA, however, little is known in the L2 writing domain regarding the influence of learner factors in L2 writing achievement. Therefore, in this study, I aimed to extend the IDs research in L2 writing by closely examining the interaction between students' cognitive abilities, motivational levels, and emotional experiences on one hand and their L2 writing achievement on the other. To that end, I drew on the latest theoretical developments in the field of SLA to conceptualize students' motivational orientations and emotional experiences during L2 writing and explore how those affective variables as well as their L2 learning abilities contribute to their success in L2 writing. A total of 85 university students learning English as a Foreign Language (EFL) at a private university participated in the study by filling out several questionnaires and taking a language aptitude test. The students were also asked to write an essay about a specific topic. Factor analytic results substantiated the construct validity of the L2 writing selves scales. In addition, reliability analyses confirmed the internal consistency of all the motivational, emotional, and cognitive measures. Two raters also graded the students' essays both holistically and analytically in terms of complexity, accuracy, and fluency. To answer the research questions, a series of multiple regression were conducted. The results showed that the ideal L2 writing self positively predicted L2 writing joy, motivation, and achievement, whereas ought L2 writing self positively predicted L2 writing anxiety and motivation. Although both L2 writing enjoyment and anxiety predicted L2 writing motivation positively, L2 writing anxiety predicted L2 writing course grade negatively. Aptitude was also found as a positive predictor of L2 writing achievement. Considering the CAF measures, only ideal L2 writing self predicted the fluency. Overall, the findings provided a clear picture on the role of IDs in the development of L2 writing. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]