Syntactic and linguistic complexity have been studied extensively in applied linguistics as indicators of linguistic performance, development, and proficiency. Recent publications have equally highlighted the reductionist approach taken to syntactic complexity measurement, which often focuses on one or two measures representing complexity at the level of clause-linking or the sentence, but eschews complexity measurement at other syntactic levels, such as the phrase or the clause. Previous approaches have also rarely incorporated measures representing the diversity of syntactic structures in learner productions. Finally, complexity development has rarely been considered from a cross-linguistic perspective, so that many questions pertaining to the cross-linguistic validity of complexity measurement remain. This article reports on an empirical study on syntactic complexity development and introduces a range of syntactic diversity measures alongside frequently used measures of syntactic elaboration. The study analyzed 100 English and 100 French second language oral narratives from adolescent native speakers of Dutch, situated at 4 proficiency levels (beginner-advanced), as well as native speaker benchmark data from each language. The results reveal a gradual process of syntactic elaboration and syntactic diversification in both learner groups, while, especially in French, considerable differences between learners and native speakers reside in the distribution of specific clause types.