Researchers and educators routinely call for longitudinal research on language learning and teaching. The present volume explores the connection between longitudinal study and advanced language capacities, two under-researched areas, and proposes an agenda for future research. Five chapters probe theoretical and methodological reflections about the longitudinal study of advanced L2 capacities, followed by eight chapters that report on empirical longitudinal investigations spanning descriptive, quasi-experimental, qualitative, and quantitative longitudinal methodologies. In addition, the co-editors offer a detailed introduction to the volume and a coda chapter in which they explore what it would take to design systematic research programs for the longitudinal investigation of advanced L2 capacities. The scholars in this volume collectively make the argument that second language acquisition research will be the richer, theoretically and empirically, if a trajectory toward advancedness is part of its conceptualization right from the beginning and, in reverse, that advancedness is a particularly interesting acquisitional level at which to probe contemporary theories associated with the longitudinal study of language development. Acknowledging that advancedness is increasingly important in our multicultural societies and globalized world, the central question explored in the present collection is: How does learning over time evolve toward advanced capacities in a second language?.