An experiment is reported examining the role of working memory in two laboratory-based prospective memory (PM) tasks. Participants viewed a film for a later recognition memory task while simultaneously monitoring auditorially presented arithmetic problems for incorrect solutions. The arithmetic verification task was either low demand or high demand. In addition, participants were required either to indicate whenever an animal appeared in the film (event-based PM task), or whenever 3 min had elapsed (time-based PM task). PM performance was higher when the arithmetic task was low demand than when it was high demand. Young participants were more successful in both PM tasks than older participants, but only under high demand. Age did not interact with PM task type overall, and the young participants were faster overall in both types of PM task. Taken together, the results indicate that working memory plays an important role in PM tasks.