This paper examines debt structure using a new and comprehensive database on types of debt employed by public U.S. firms. We find that 85% of the sample firms borrow predominantly with one type of debt, and the degree of debt specialization varies widely across different subsamples—large rated firms tend to diversify across multiple debt types, while small unrated firms specialize in fewer types. We suggest several explanations for why debt specialization takes place, and show that firms employing few types of debt have higher bankruptcy costs, are more opaque, and lack access to some segments of the debt markets.