A comparison challenge approach is proposed as a form of challenger-activated, just-in-time advertising. To develop a framework for a comparison challenge, we propose a theory of comparison. Based on this theory, the CompareMe and CompareThem strategies are devised, and comparable objects are classified in terms of price and performance dominance as well as the scope of proximity. The idea is demonstrated with a comparison of PCs from five leading manufacturers. To assist in the planning of the comparison challenge, a mathematical programming model was formulated to maximize the value of comparison under the constraints of the comparison opportunity and budget. The model is applied to eight scenarios in terms of the range of comparing objects. We found the ad effect of comparison challenge to be substantially better than banners (4.75 times) and similarity-based comparisons (2.77 times), providing customers with better performance and reduced prices.