Much recent scholarship has sought to identify the linguistic and social factors that favor the expression or omission of subject pronouns in Spanish. This volume brings together leading experts on the topic of language variation in Spanish to provide a panoramic view of research trends, develop probabilistic models of grammar, and investigate the impact of language contact on pronoun expression. The book consists of three sections. The first studies the distributional patterns and conditioning forces on subject pronoun expression in four monolingual varieties - Dominican, Colombian, Mexican, and Peninsular - and makes cross-dialectal comparisons. In the second section, experts explore Spanish in contact with English, Maya, Catalan, and Portuguese to determine the extent to which each language influences this syntactic variable. The final section examines the acquisition of variable subject pronoun expression among monolingual and bilingual children as well as adult second language learners.