An examination of 92 languages which resolve hiatus through Vowel Elision and/or Coalescence (merger) reveals two correlations that pose interesting challenges for phonological theory. First, in certain contexts the vowel targeted by Elision is universally predictable. At (non-function) word boundaries for example, languages consistently elide the leftmost (word-final) vowel. Second, the type of Coalescence possible in a language depends on the language's vowel inventory. While seven-vowel systems coalesce /a+i/ to [E], for example, nine-vowel systems coalesce this sequence to [e]. These generalizations are analyzed within the framework of Optimality Theory. My analysis of Elision assumes that languages make greater effort to preserve features occurring in certain phonetically or semantically prominent positions (e.g. in roots). Corresponding to these are a series of position-sensitive Faithfulness constraints requiring preservation of features in these positions. The possible rankings of these constraints yield an Elision typology in good agreement with attested patterns. The Coalescence correlations strongly suggest that the height features of a vowel depend on the inventory in which the vowel occurs, a claim supported by other facts as well. I propose that (auditorily defined) height features are assigned to vowel systems via best satisfaction of a set of constraints on height specification, for example a constraint requiring minimal use of certain features. The specifications assigned to a given vowel system will, in conjunction with a uniform ranking of Faithfulness constraints that characterizes Height Coalescence across all the systems, correctly generate the possible patterns in that type of system. In its prototypical form, Elision is position-sensitive: the elided segment is the one that occupies a particular position. (Symmetric) Coalescence, on the other hand, is feature-sensitive: the resulting vowel retains the most highly-preferred features from the original vowels. Also attested are a "Feature-Sensitive" Elision process in which the vowel preserved is the one which possesses particular feature(s), and a type of "Asymmetric" Coalescence, in which positional and featural preferences both play a role. All four processes are correctly generated by the analysis, arising under different permutations of the same constraints posited to account for Elision and Coalescence in their prototypical incarnations.