This volume explores the multiple aspects of morphological complexity, investigating primarily whether certain aspects of morphology can be considered more complex than others, and how that complexity can be measured. The book opens with a detailed introduction from the editors that critically assesses the foundational assumptions that inform contemporary approaches to morphological complexity. In the chapters that follow, the volume's expert contributors approach the topic from typological, acquisitional, sociolinguistic, and diachronic perspectives; the concluding chapter offers an overview of these various approaches, with a focus on the minimum description length principle. The analyses are based on rich empirical data from both well-known languages such as Russian and lesser-studied languages from Africa, Australia, and the Americas, as well as experimental data from artificial language learning.