The book investigates cognitive development in young children through a series of functional near-infrared spectroscopy (fNIRS) studies conducted by the author and his team. It explores the neural correlates of cognitive processes, such as mental rotation, inhibitory control, and joint attention, shedding light on the mechanisms underlying these processes in preschoolers. Methodological advancements in fNIRS research, including multivariable and multiscale sample entropy analysis, are highlighted to demonstrate cutting-edge approaches in studying neurocognitive development. Furthermore, the book examines the influence of sociocultural and linguistic factors on cognitive processes. Through a multidisciplinary approach that integrates developmental psychology, neuroscience, and education, it offers a comprehensive perspective on neurocognitive development in preschoolers, contributing to the evolving discourse on early childhood cognitive functioning and laying the groundwork for future research directions. The book will appeal to scholars of developmental psychology, neuroscience, and education. It also provides practical implications for educators, policymakers, and practitioners working with preschoolers.