This dissertation sets out a theory of phonology-morphology interaction consistent with parallelist Optimality Theory. The core idea is that word formation rules -- e.g., affixation of _cat_ to yield _cats_ -- are mirrored by an identity relation between the derived word and its base word. This phonological relation, which holds between two surface forms, is called output-to-output (OO) correspondence, and is conceived as part of the Correspondence Theory of faithfulness proposed by McCarthy & Prince (1995). Thus, like input-to-output (IO) faithfulness, OO-faithfulness is regulated by ranked and violable constraints in a monostratal grammar.OO-faithfulness competes with IO-faithfulness (and with markedness constraints) in the optimization of pairs of related words, or subparadigms.This theory is motivated by a class of cases in which identity of related words surpasses what is expected from shared underlying form.In these cases, a derived word violates a phonotactic pattern to better resemble its base word -- e.g., _c"ond"ens'ation stresses its second syllable, and violates a constraint against stress clash, to achieve identity with its base _c"ond'ense_. Previous analyses of these patterns rely on serial ordering, allowing _c"ond'ense_ to serve as an intermediate stage in the derivation of _c"ond"ens'ation_.OO-correspondence obviates serialism, explaining so-called "cyclic effects" as the product of constraint ranking in fully parallel derivation.