Abstract: I review the empirical comparative law literature with an emphasis on quantitative work. After situating the field and surveying its main applications to date, I turn to methodological issues. I discuss at length the obstacles to causal inference from comparative data, and caution against inappropriate use of instrumental variables and other techniques. Even if comparative data cannot identify any single causal theory, however, they are extremely important in narrowing down the set of plausible theories. I report progress in measurement design and suggest improvements in data analysis and interpretation using techniques from other fields, particularly growth econometrics.