Using a comprehensive and unique data set from Moody's, we examine correlations between default risk for over 7,000 U.S. public firms. This is the first paper to empirically document the correlation structure both in the time-series and in the cross-section across almost all U.S. non-financial firms. We find that default probabilities of issuers vary over time, and are positively correlated. Moreover, the correlations across firms also vary over time systematically, in a manner that is related to an economy-wide level of default risk. Joint default risk increases as the default risk in the economy increases. Our results also suggest that the magnitude of joint default depends on the quality of issuers; highest quality issuers have higher default correlations than medium grade firms.