This paper aims to understand the impact of temporal spacing between ad exposures on the likelihood of a consumer purchasing the advertised product. I create an individual-level data set with exogenous variation in the spacing and intensity of ads by running online field experiments. Using this data set, I first show that (1) ads significantly increase the likelihood of the consumers purchasing from the advertiser and (2) this increase carries over to future purchase occasions. Importantly, I find evidence for the spacing effect: the likelihood of a product’s purchase increases if the product’s past ads are spread apart rather than bunched together, even if the spreading apart of ads involves shifting some ads away from the purchase occasion. Because the traditional models of advertising do not explain the data patterns, I build a new memory-based model of how advertising influences consumer behavior. Using a nested test, I reject the restrictions imposed by the canonical goodwill stock model based on the Nerlove and Arrow  approach, in favor of the more general memory-based model. Counterfactual simulations using the parameter estimates show that not accounting for the features of the memory model might lead to significantly lower profits for the advertisers.