This chapter provides a comprehensive overview of basic laboratory research on resurgence, a form of behavioral relapse in which previously extinguished responses reemerge following the degradation of reinforcement for alternative behavior. The authors trace the historical development of resurgence procedures and highlight the role of nonhuman animal models in identifying key mechanisms of relapse. Common experimental arrangements and dependent measures—such as response rates and cumulative responses—are discussed in detail. The chapter also reviews variables influencing resurgence, including reinforcement rate, magnitude, quality, and delays, as well as response parameters such as effort and topography. Several conceptual frameworks, including behavioral momentum theory, context theory, and resurgence-as-choice models, are presented to explain the behavioral processes underlying resurgence.