This dissertation contributes to the study of interest groups by connecting a key structural characteristic of interest groups to the effectiveness of group lobbyists. The connection occurs through what I term interest group markets. By interest group markets I mean collections of interest groups working on the same side of the same policy area. Concentration is a structural characteristic of a market. Markets with few organizations or one dominant or- ganization are highly concentrated, while those with many groups of roughly the same size are characterized by low concentration. While economic theory suggests that competitive markets are usually more efficient, I argue that highly concentrated interest group mar- kets may be more effective at changing policy outcomes. Concentrated groups benefit from economies of scale and low transaction costs. The first article in my dissertation examines concentration in pro-life interest group markets in all 50 states and attempts to predict the level of concentration in each market. The second article tests my hypothesis that more concentrated markets will be more successful at influencing policy outcomes.The third article addresses a measurement problem related to the study of interest group influence on political outcomes. I discuss a source of error in expenditure reports requiredby the Lobbying Disclosure Act of 1995. Researchers who have used this data in the past appear to be entirely unaware of the problem. I demonstrate the extent of the problem using an original dataset of sampled groups. Then, I replicate a working paper and simulate the effect of this error on the papers results. This demonstration shows that in some cases using LDA data may be misleading.