What will increase individuals' compliance with the federal income tax? There are rich legal, economic, and sociological literatures examining this question. The traditional answer that increased enforcement will increase compliance is supported by both economic modeling and a number of experiments. However, studies show that appeals to normative beliefs about honesty in taxpaying play an important role as well. A number of scholars have suggested that vigorous enforcement of the tax laws may be counterproductive because it may suggest that noncompliance is the norm. This article argues, in part, that enforcement and a compliance norm are not inconsistent but rather are complementary. In other words, enforcement can buttress norms-based appeals for compliance. To support this argument, the article draws on an array of empirical evidence from both experimental "games" conducted in the laboratory and field experiments involving taxpayers. The interplay of enforcement and taxpaying norms manifests itself somewhat differently in different contexts. Studies suggest that there is a societal norm of compliance with tax obligations but that there may be a norm of noncompliance among certain groups. The IRS may therefore be best served by targeted compliance strategies. With respect to mainstream, generally compliant taxpayers, the IRS can rely on broad-based matching of information returns with taxpayer returns, low levels of audits, and norms-based appeals. With respect to groups with norms of noncompliance, the IRS can use enforcement not only for detection and deterrence but also to try to build a critical mass of compliant taxpayers and thereby influence the group norm.