When in the late 1990s we set out to review the accumulated research on second language (L2) instructional effectiveness, it was with a sense – though no certainty – that a journey into the uncharted territory of systematic research synthesis would eventually prove worth the effort. In order to find a fitting methodology, and in the absence of any guidance within applied linguistics, we willingly delved into challenging techniques and debates from other disciplines. A few years later, we had to search long and hard to find enough synthesists working on language learning and teaching issues to warrant a book, but we did, and we saw that new effort as an important, if initial, step towards the adoption and adaptation of research synthesis, and particularly the subset of methods known as meta-analysis, to the problems of applied linguistics. Now, some ten years after our first publication on the topic, applications of meta-analysis have increased dramatically in the field, as this Timeline clearly shows. For some topics (e.g. interaction, corrective feedback), we have even seen several studies replicating and building on each other. We are pleased with these developments and welcome the efforts of the field to engage in rigorous and meaningful reviews of L2 research, with an eye towards cumulative explanation of key phenomena. At the same time, we also hope that researchers will avail themselves of the full potential of systematic research synthesis, which considerably transcends the narrower domain of meta-analysis and includes other quantitative as well as qualitative methods that more fully enable the task of synthesizing accumulated knowledge in the increasingly diverse research landscape of applied linguistics.