Linguistic complexity serves as an important measure of second language (L2) writing development. Complexity indices, however, rarely feature in the studies of learner languages other than English. Additionally, L2 writing studies have been criticized for the lack of consistency in defining proficiency. The current study addresses these gaps by exploring writing development in a less commonly studied language (i.e., Russian) while paying attention to the operationalization of proficiency. We analyzed lexical and syntactic complexity indices in a corpus of essays written by learners at different levels at the beginning and the end of an instructional program. Our analyses demonstrate that 9 complexity indices (i.e., mean word length, type--token ratio, percentage of high-frequency words, mean sentence length, clauses per sentence, syntactic depth, proportion of subordinate clauses, and proportion of relative clauses) changed significantly over the course of the program; the same 9 indices showed significant correlation with the initial curricular placement, and all--with the exception of clauses per sentence--modestly or highly correlated with the final rating on the proficiency test. The findings confirm the usefulness of complexity measures in tracking writing development in L2 Russian.