This book presents an in-depth investigation of the developmental path of English as a second language (ESL) acquisition. Understanding the natural acquisition process benefits syllabus construction and task designs in language education as formal instruction can be more efficient if the learner is developmentally ready. However, a majority of second language acquisition (SLA) studies are cross-sectional, and studies observing second language development over time, particularly those for more than one year, are very rare. This book, based on a two-year longitudinal study of a Japanese primary school child learning English in Australia, explores the staged development of English morphology and syntax within the framework of Processability Theory (PT), which is one of the current major SLA approaches. The research in this book provides valuable evidence supporting the PT hypotheses. It also addresses the issues of the relationship between morphological and syntactic development. The findings in this book contribute to the development of SLA studies as well as ESL/EFL teaching. This book is recommended for all in the field of SLA, child language, and ESL/EFL education.