The National Institute of Justice's Multi-site Adult Drug Court Evaluation (MADCE) tested whether drug courts reduce drug use, crime, and associated problems; assessed how drug courts work and for whom; examined how changes in participant attitudes and behaviors explain effectiveness; and determined whether drug courts generate cost savings. The evaluation found that drug courts prevent crime and substance use and work equally well for most participant subgroups. Effects are greatest among participants whose judges who spend time with them, support them, and treat them with respect. Methodology and implications for policy and practice are also discussed.