While the comparative method is concerned with regularities in phonological change, grammaticalization theory deals with regularities of grammatical change. In an A-Z format, this book summarizes the most salient generalizations that have been made on the unidirectional change of grammatical forms and constructions. It thus demonstrates that grammatical change is regular and also adds to our knowledge of universal properties of human languages. Indices organized by source and target concepts allow for flexible use, and the findings delineated in the book are relevant to students of language across theoretical boundaries.