Scholars have avoided studying interest group influence because of the difficulty operationalising the concept. The research presented here introduces a new way of measuring lobbying success and lays out a model of its determinants. To understand why interest groups sometimes succeed and at other times fail, we must consider the institutional structure of the political system within which the advocates are operating; the characteristics of the issue at hand; and finally the characteristics of the interest group itself and their lobbying strategy. I test these factors with original data based on interviews with 149 advocates in Washington D.C. and Brussels active on a random sample of 47 policy issues. From the results, issue context emerges as a much more important determinant of lobbying success than institutional differences. The institutional differences that do emerge suggest that direct elections coupled with private campaign finance lead to winner-take-all outcomes biased in favour of wealthier business interests, while the lack of these institutional characteristics leads to more balanced policy compromises with more advocates achieving some of their policy goals.