This collection of nine papers brings together Naoki Fukui’s pioneering body of work on Merge, the basic operation of human language syntax, from the two distinct but related perspectives of theoretical syntax and neurosciences. Part I presents an overview of the development of the theory of Merge and its current formulations in linguistic theory, highlighting the author’s previously published papers in theoretical syntax, while Part II focuses on experimental research on Merge in the brain science of language, demonstrating how new techniques and the results they produce can inform the study of syntactic structures in the brain in the future. By combining insights from theoretical linguistics and neurosciences, this book presents an innovative unified account of the study of Merge and paves new directions for future research for graduate students and scholars in theoretical linguistics, neuroscience, syntax, and cognitive science.