Expression and omission of subject personal pronouns has been studied extensively in various perspectives and using terminology ranging from “pro-drop” in generative linguistics to “variable subject expression” in functional approaches. Traditional grammar considers null subjects the norm and refers to “contrast” and “emphasis” to account for pronoun expression. Genuinely contrastive uses of subject pronouns are nevertheless rare and the choice between expressing the subject only by means of bound person marking (canto ‘[I] sing’) or by both independent and bound person marking (yo canto ‘I sing’) depends on various factors. While there is extensive evidence that grammatical person and coreferentiality with previous subjects affect subject expression in all varieties of Spanish, frequency of subject expression varies across dialects, typically being highest in Caribbean and lowest in Mexican and Peninsular varieties. Priming of previously used sequences and entrenchment of frequently repeated verb-subject combinations have also been shown to account for variable subject expression in spoken discourse. The current chapter sets out by defining the scope of variation and situating Spanish in a language-typological perspective. Next, an overview is provided of the factors that have been found to affect the phenomenon. Finally, we summarize the findings and discuss directions for future research.