As part of its semiannual meeting in Washington DC, the National Academies' Committee on Law and Justice (CLAJ) held a half-day seminar on legal cannabis for the benefit of our sponsor, the National Institute of Justice, and other interested stakeholders. The seminar focused on research opportunities presented by the current changes and legalizations, and unmet research needs for sound decision-making. A panel of experts provided background on the regulatory issues around legal cannabis and the available scientific evidence and examined the similarities and contrasts with alcohol and tobacco.