This supplement issue on the treatment of marijuana use disorders describes two large multi-site field experiments: the Cannabis Youth Treatment (CYT) study with adolescents and the Marijuana Treatment Project (MTP) with adults. The papers cover multiple aspects of the treatment of cannabis users, including the rationale for studying cannabis use disorders, descriptions of the CYT and MTP studies,characteristics of adolescents and adults presenting for treatment of cannabis use disorders, court diversion issues, economic evaluation and confirmation of self-reported cannabis use, among other topics. This Introduction provides background information and an overview of the papers from the perspective of the funding agency.