This special issue, focusing on different types of variation in syntactic complexity, offers a critical discussion of the role of variation in syntactic complexity research in SLA. Special attention is paid to inter‐learner variation (i.e., individual developmental trajectories of acquiring syntactic complexity) and to interactions with related constructs, for instance between syntax and morphology, and between syntactic and lexical complexity. The overarching focus of the different contributions is to investigate the possible variation of (syntactic) complexity according to various external factors, such as levels of linguistic proficiency (developmental variation), variation in task modality (i.e., oral, written), task types and genres, and variation in source and target languages (cross‐linguistic variation

This study builds upon previous research investigating the construct validity of phraseological complexity as an index of L2 development and proficiency. Whereas previous studies have focused on cross-sectional comparisons of written productions across proficiency levels, the current study compares thelongitudinaldevelopment of phraseological complexity in writtenand oralproductions elicited over a 21-month period from learners of French. We also improve upon the state of the art by including L1 data to benchmark learner levels of phraseological complexity. Phraseological complexity, operationalized as the diversity (no. types) and sophistication (PMI) of adjectival modifiers (adjective + noun) and direct objects (verb + noun), was generally higher in learner writing as compared to speaking. Over the study period, the sophistication of phraseological units increased slightly but developmental patterns were found to differ between tasks, highlighting the importance of considering task characteristics when measuring phraseological complexity.