Political scientists have developed accepted numerical estimates of political ideology for legislators, candidates, parties and even nations, but interest groups lack such scores. This absence puts interest group research at a disadvantage compared to other subfields. I generate ideology scores for 72 groups across 10 years by combining groups’ evaluations of Members of Congress with Poole and Rosenthal’s estimates of Members’ ideologies. Alternative methods are explored, and the validity of the scores is demonstrated. Examinations of the scores focus on the relative distribution of groups and Members of Congress and the link between a group’s ideology and its campaign contributions.