In Nguni languages (Ndebele, SiSwati, Xhosa and Zulu), ignoring penultimate lengthening and some long vowels in Xhosa, a vowel + vowel (VV) sequence occurring within a single phonological word is unacceptable. This paper focuses specifically on this problem, providing various situations where it occurs and examining the strategies used by the Nguni languages in resolving it. It is demonstrated that ill-formed sequences have to be repaired through at least one of the three main vowel processes: coalescence, gliding (consonantalization), and deletion. While the data can be analyzed within the Optimality Theory (OT) framework, it requires more than just input-output correspondence, as there is an important intermediate stage which cannot be ignored, particularly where vowel gliding triggering palatalization of a preceding consonant occurs. It is argued that while the Output-Output Correspondence Theory (Benua 1995, 1997b, Kager 1999, and Kenstowicz 1996) and the Sympathy Theory (McCarthy 1999, 2003), or some modified version of these (Coetzee 2002), can lead to the desired result, they do not provide an explanation about the crucial intermediate stage. The author concludes that Stratal OT (Kiparsky 2000), a fusion of Lexical Phonology and OT, is best suited to handle opacity, particularly the type displayed by Nguni data.