This book examines the operation of laws, rules, and principles in Indo-European, the language family which includes the Celtic, Germanic, Italic/Romance, and Baltic/Slavic subfamilies as well as the predominant languages of Greece, Iran, parts of Southern Asia, and ancient Anatolia. Laws and rules are crucial to Indo-European studies: they constrain the reconstructions and etymologies on which knowledge of the history and prehistory of Indo-European in particular and ancient languages more generally is based, and which allow processes of morphological change, semantic shift, and borrowing to be identified. But these laws and rules require constant reassessment in the light of new evidence, theory, and method. Through a series of case studies re-examining specific laws and rules in the Indo-European language family, this book explores the implications of new insights into language change andof increasing opportunities for attention to chronology and detail in the treatment of primary material. The languages and language families under consideration include Celtic, Germanic, Italic and Romance, Armenian, Greek, and Indo-Iranian languages as well as Proto-Indo-European. Laws and Rules in Indo-European brings together leading scholars from all over the world. It makes a valuable contribution to the understanding of the history of ancient languages and the reconstruction of their ancestors, as well as to research methods.