A system and method for placing ads on a client-side video replay system. The described embodiments of the present invention allow a client to make a determination of ad placement priority separately from the function of serving ads to a requesting application. A described dynamic ad placement engine reorders ads when the context of the system is changed to yield a next ad to be placed. Thus, at a later time, when an application sends an ad request, the dynamic ad placement engine can quickly respond with the ad to be placed. The ads also are preferably reordered after an ad is placed. Placement of ads is controlled in the described embodiment by interpreted rules for each ad.