The present chapter aims to provide readers with an overview of the seminal and current research concerning the Spanish vowel system. The basic articulatory and acoustic properties of Spanish vowels are introduced first, followed by an in-depth description of the influence of linguistic and stylistic factors (i.e., syllable structure, lexical stress, and speech style, respectively) on their production. Dialectal variation among Spanish vowel systems is summarized next, including a review of cross-dialectal comparisons of the full system as well as region-specific phenomena. After the discussion of regional variation, the impact of sociolinguistic factors such as gender, sexual orientation, social class, and social networks on the pronunciation of the Spanish vowels is examined. The final major section focuses on bilingual vowel systems, with particular emphasis on Spanish vowels produced by Spanish-English balanced bilinguals, heritage speakers of Spanish, and second language learners of Spanish. The chapter closes with suggested directions for future research and some general conclusions.