In the past three decades, the construct of second language (L2) writing complexity has been theorized and refined in both second language acquisition (SLA) (Crossley, 2020; Housen, De Clercq, Kuiken, & Vedder, 2019; Lu, 2011; Norris & Ortega, 2009) and Systemic Functional Linguistics (SFL) research (Byrnes, 2009; Ryshina-Pankova, 2015; Schleppegrell, 2004). The general consensus is that lexical and syntactic variations are regarded as signs of advanced academic writing. The contemporary legal writing pedagogy, however, is informed by the Plain English Movement (Benson, 1985; Dorney, 1988; Felsenfeld, 1981), which largely discourages the use of overly complex structures and "elegant variations." The recommendation to use plain English in legal writing thus poses a challenge to the theoretical consensus in SLA and SFL research and raises a question about the conceptualizations and assessment of writing complexity in academic legal writing classrooms. This dissertation consists of three interrelated studies and aims to address this contradiction by examining the development and assessment of writing complexity (i.e., lexis, syntax, and discourse) in 246 hypothetical legal essays written by 31 international (LL.M.) students over one-year of legal language study at the Georgetown University Law Center. In Study 1, I used a structural, corpus-based approach and tracked the changes of 31 students' lexical and syntactic complexity in six data collection points over one year and compared the complexity indices with those benchmarked by eight model essays. In Study 2, I offer an in-depth discussion of four students' distinct developmental trajectories of discourse complexity, which I analyzed through the system of "engagement" (Martin & White, 2005) from the SFL perspective. Finally, in Study 3, I adopted a mixed-methods approach to investigate two legal instructors' conceptualizations of writing complexity in the classroom setting. Results showed that overall students were able to write with significantly more sophisticated words and, in the second semester, significantly shorter sentences, a pattern consistent with the pedagogical focus of the program. Additionally, the four individual trajectories of discourse complexity indicate that, even when starting with a similar proficiency level, some students constructed increasingly complex legal discourse by actively engaging different legal voices through dialogic expansion and contraction, while others only made marginal progress. Consistent with the instructors' conceptualizations of writing complexity, structurally more complex essays were not necessarily of higher text quality. In fact, the instructors' assessment of text quality was found to be influenced by a number of external factors, such as students' performance in comparison to others and their academic progress in the program. I conclude by discussing some additional insights that emerged from the dissertation such as the boundaries between text modeling and plagiarism, as well as the role of text length in assessing timed writing. Finally, I call for a critical reflection on the pedagogical principles of Plain English and highlight the value of an integrated structural-functional approach to holistically understand the construct of L2 writing complexity in academic legal 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.]