It is not uncommon for previously reinforced, but thereafter eliminated or abandoned respondent or operant behavior to recur. As the articles in this special issue of the Mexican Journal of Behavior Analysis attest, such recurrence has implications for both the theoretical understanding of behavior and application. Perhaps the first type of recurrence to be investigated was spontaneous recovery, which occurs after a period of nonexposure to the context in which the response was extinguished (e.g., Pavlov, 1927). Later, other circumstances of recurrent behavior were discovered, and theoretical infrastructures developed to not only account for recurrence, but also to integrate these circumstances according to their commonalities. Observing and harnessing the understanding of these recurrence phenomena for application developed concurrently.