The first cross-linguistic study of infixation explores its prosodic, phonological, and morphological characteristics, considers its diverse functions, and formulates a general theory to explain the rules and constraints by which it is governed. It examines 154 infixation patterns from over a hundred languages, compares their formal properties and explores their diachronic origins. The book will interest phonologists, morphologists, typologists, and historical linguists of all theoretical persuasions.