This book is about how the vocabulary of a language is built and how similarities and differences between word forms can be explained, integrating optimality theory (OT) with lexical phonology and morphology. It shows that the phonology and morphology of stems, words, and sentences is governed by distinct constraint systems which may differ in the ranking of faithfulness constraints, and that these constraint systems are serially linked. Parallel versions of OT with paradigmatic constraints are refuted. The book includes a systematic comparative analysis of Arabic dialects. It presupposes some knowledge of phonological theory.