Optimality Theory (OT) has been a major force driving developments in formal linguistics during the past decade. Like parameter-setting accounts, OT seeks to describe the range within which languages can vary; but instead of fixing parameters, OT proposes that languages, and children learning languages, arrange a set of constraints in a hierarchical order of strength that determines specific linguistic characteristics. The constraints proposed by OT are constraints on the well-formedness of the output of a grammar, and they are of two types: (i) markedness constraints, which exert pressure toward unmarked types of structure such as CV syllables or voiceless final obstruents; and (ii) faithfulness constraints, which maintain lexical contrasts such as CV and CVC syllable types, or voicing distinctions in final obstruents. The two types of constraints are in conflict, so that no particular constraint can be satisfied without violating others. In OT, “satisfaction” and “violation” are not absolute but a matter of degree, because all constraints play a role in the grammar of each language, though in a different order of strength or priority within a dominance hierarchy.