Task-based language teaching (TBLT) methodology is theoretically and empirically motivated. It is rooted in in cognitive and interactionist second language acquisition theory and based on inferences from research findings that show how learners acquire an second language. As a result, TBLT is able to create a psycholinguistically optimal environment for second language learning by taking into account the main factors that contribute to the sedond language learning process and that make seond language teaching more efficient. This chapter summarizes the theoretical underpinnings of TBLT methodology and provides a review of major research findings on variables in instructed second language learning that have been identified as having a relevant impact on second language learning: attention to form, negative feedback, and cognitive individual differences. The chapter concludes by discussing the implications of the research findings presented for a psycholinguistically defensible methodology for TBLT and the problems these findings pose for traditional options in language teaching, such as the structural syllabus.