In this thesis we will analyse phonological phenomena which involve nuclear positions in Brazilian Portuguese. The theory of Charm and Government is the framework on which our analysis is based. We will discuss the distribution of Brazilian Portuguese vowels with respect to the primary stressed position which, we propose, is subject to charm constraints imposed on nuclear segments. Emphasis is placed on analysing vowel coalescence in Brazilian Portuguese. Specifically, we will consider phonological processes which involve vowel- glide alternations. We will argue that whether a high vowel alternates or fails to alternate with its corresponding glide depends on the governing relations that adjacent nuclear positions contract with each other. Our analysis aims not only to provide a general account for the occurrence of high vowels and glides, but also to determine how glides are phonologically interpreted in Brazilian Portuguese.