This volume honors the scholarly legacy of Kimberly L. Geeslin. Geeslin’s pioneering work on variation in the Spanish copula system united and extended research in the fields of second language acquisition and sociolinguistics. Geeslin laid the foundation for a growing subfield of investigation that explores how interlanguages vary in systematic and socially meaningful ways across various modules of language; how variation in learner language relates to the speakers, contexts, and experiences learners are in contact with; and how variable features develop over learning trajectories. This volume connects established and up-and-coming scholars conducting research on second language sociolinguistic variation and exemplifies the present and future of this line of inquiry. Together, these chapters reconsider important questions, pose and test new methods, and challenge long-standing practices to advance both second language acquisition and sociolinguistics. The volume’s collaborative format also pays homage to Kim Geeslin’s unparalleled mentorship and field-wide influence.