This dissertation is concerned with the Obligatory Contour Principle (OCP) and its relationship to the representation of geminate consonants. The OCP blocks lexical forms with pair geminates, a pair of adjacent identical melodies. Therefore geminates must be represented as single melodies associated to two timing units. The OCP is also active on outputs, blocking phonology from creating pair geminates. The dual nature of the OCP (as both input and output constraint) is derived from the interaction of ranked and violable output constraints in an Optimality-theoretic grammar. In this analysis, no input restrictions are required. The OCP is interpreted as a constraint on the set of constraints in UG (CON). The lexical OCP is accounted for by positing that no faithfulness constraint requires maintaining a distinction between one segment and two identical adjacent segments. The output OCP is accounted for by positing that output markedness constraints universally prefer one segment to two. The interaction of these markedness and faithfulness constraints neutralizes the contrast between pair and single geminates. One consequence of the analysis is that no specific OCP constraint is required. Rather, the effects of the OCP follow from general markedness considerations. Geminates behave differently with respect to phonological changes compared to their singleton counterparts. Geminates are sometimes affected by changes that affect singletons (alterability). Examples of geminate alterability are found in Faroese, Persian, Fula, and Alabama. The fission of geminates appears to be a counter example to the claim that markedness universally prefers one segment to two. It is shown that fission follows from the activity of faithfulness constraints relativized to the syllable onset. The analysis of fission captures an asymmetry in fission processes. No fission process creates a cluster where the initial segment is more faithful to the input than second segment. In addition to alterability, geminates are sometimes unaffected by changes that affect singletons (inalterability). Examples of geminate inalterability include Tiberian Hebrew, Latin, and the restriction of coda consonants in many languages. Universal inalterability must be an effect of the constraint responsible for the change in singletons. Parochial inalterability however, is the result of standard constraint interaction in an OT grammar.