Governments should embrace randomized trials to estimate the efficacy of different laws and regulations. Just as random assignment of treatments is the most powerful method of testing for the causal impact of pharmaceuticals, randomly assigning individuals or firms to different legal rules can help resolve uncertainty about the consequential impacts of law. We explain why randomized testing is likely to produce better information than nonrandom evaluation of legal policies and offer guidelines for conducting legal experimentation successfully, considering a variety of obstacles, including ethical ones. Randomization will not be useful for all policies, but once government gains better experience with randomization, administrative agencies should presumptively issue randomization impact statements justifying decisions to implement particular policies. Making the content of law partially contingent on the results of randomized trials will promote ex ante bipartisan agreements, as politicians with different empirical predictions will tend to think that the experiments will support their position.