This article is a first attempt to systematically examine policy design and its influence on policy effectiveness in a comparative perspective. We begin by providing a novel concept and measure of policy design. Our Average Instrument Diversity (AID) index captures whether governments tend to reuse the same policy instruments and instrument combinations or produce policy solutions that are carefully tailored to the policy problem at hand. Second, we demonstrate that our AID index is a valid and reliable measure of policy design quality with a strong explanatory power for the outcome variables tested. Analyzing the composition of environmental policy portfolios in 21 OECD countries, we show that higher levels of AID are positively associated with a country’s policy effectiveness in environmental matters. Based on this finding, we analyze, in a third step, the factors that lead countries to adopt more or less diverse policy portfolios. We find that the policy design quality is significantly improved when policy makers are not bound by high institutional constraints and, more importantly, are backed by well-equipped bureaucracies.