This is a pre-copyedited, author-produced version of an article accepted for publication in Applied Linguistics following peer review. The version of record Ellis, R. 2015. Introduction: Complementarity in Research Syntheses. Applied Linguistics. 36 (3): pp. 285-289 is available online at: https://academic.oup.com/applij/article-lookup/doi/10.1093/applin/amv015