Syntactic complexity has been a major focus in studies of second language (L2) writing development for the past four and a half decades. Despite its frequent use, the construct remains underdeveloped, which can make it difficult to interpret the results of syntactic complexity studies. This can be attributed to at least three reasons: inconsistencies and lack of clarity in the description of syntactic complexity measures, the use of overly broad measures, and the neglect of usage-based perspectives. To address these gaps, this study introduces the Tool for the Automatic Assessment of Syntactic Sophistication and Complexity (TAASSC), which measures both large and fine-grained clausal and phrasal indices of syntactic complexity, as well as usage-based frequency/contingency indices of syntactic sophistication. By utilizing TAASSC, this study will investigate L2 writing development in two main ways: first, by examining syntactic development longitudinally, and second, by examining human judgments of writing proficiency, such as expert ratings of TOEFL essays. The results of this study will have important implications for the fields of second language acquisition, second language writing, and language assessment.

INDEX WORDS: Second language acquisition, Natural language processing, Language use, Language assessment, Syntactic complexity, Writing development