Within second language acquisition research, L2 development has been traditionally analyzed through the dimensions of Complexity, Accuracy and Fluency (CAF) (Larsen-Freeman, 2009; Ortega, 2003; Skehan, 2009). Complexity within the CAF framework has gained the most attention and has often been examined through measures associated with clausal length (Bulte & Housen, 2012). In contrast, the present study examines complexity through the register-functional framework (Biber, Gray & Poonpon, 2011; Biber, Gray & Staples, 2016). The fundamental principle of the register-functional framework is that complexity is situation-dependent, meaning that complexity depends on the situational characteristics of texts such as communicative purposes of texts and their production circumstances. Thus, instead of relying on particular measures of complexity such as complexity indices or T-unit measures, the investigation of complexity within the register-functional framework begins with a linguistic description of texts. The present study builds on the foundation of the register-functional framework and adds to the body of L2 development research by being the first of its kind in L2 Russian. Previous studies that investigated L2 complexity in Russian are rather few (e.g., Henry, 1996; Kisselev & Alsufieva, 2017) but are also limited in that they 1) only study writing; 2) use either omnibus complexity measures or a rather limited set of measures; 3) investigate high levels of proficiency. The overall goal of this study is to provide a comprehensive description of syntactic and morphological L2 development at lower levels (e.g., beginner to intermediate) across speech and writing in Russian. To address these research gaps, the present study examines L2 development through morphological and syntactic complexity measures in L2 Russian. The study uses a corpus of written and spoken texts produced by learners across four program levels (i.e. the first two years of Russian). First, the study examines individual measures of morphological and syntactic complexity and interprets the findings in light of the curriculum progression and assignment effects. Second, the study performs a Multidimensional Analysis (MD) in order to group the individual measures of complexity into dimensions of complexity that are interpreted functionally. The results of the study show that both syntactic and morphological complexity measures behave differently across program levels. For example, while adverbial "if-" and "when-" clauses increase with program level, adverbial "because-" clauses decline. Similarly, while post-modifying nouns increase, attributive adjectives decrease. In terms of morphological complexity, the measures that have clear increasing trends across program levels are genitive nouns and adjectives, instrumental nouns and adjectives, dative nouns, and past perfective and imperfective verbs. The Multidimensional Analysis (MD) yielded two dimensions of complexity: 1) Narrative vs. Non-narrative/Descriptive, and 2) Informational vs. Personal. The narrative side of Dimension 1 includes perfective and imperfective past verbs, while the non-narrative side includes 3rd person plural verbs and attributive adjectives. The informational side of Dimension 2 is represented by complexity measures such as prepositional adjectives, genitive singular nouns, genitive adjectives and attributive adjectives. In contrast, the personal side is characterized by such measures as 1st person present tense verbs, accusative nouns and non-finite complement clauses. These dimensions of complexity showed significant differences between program levels. Significant interactions between program level and mode were also demonstrated pointing out to differences between speech and writing with regards to these dimensions across program levels. Although the complexity measures included in these dimensions are very specific to Russian, these two dimensions have been consistently identified in other MD studies. [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.]