Abstract: A method and system for billing for online ad delivery services are disclosed. According to one aspect of the invention, an advertiser establishes an account with an online ad delivery service, and provides an initial amount of funds for the account. In addition, the advertiser establishes a budget limit for a specified budget period. As ads are served and clicked by end-users, a predetermined fee is debited from the advertiser's account. However, the ad server includes budget analysis logic to determine when to stop serving ads to ensure that the advertiser's budget limit is not exceeded, thereby ensuring that the advertiser's initial funds are not used up more quickly than desired.