The Montreal Protocol established a staggered schedule for phasing out ozone-depleting substances (ODS) that unintentionally contributed to illegal trade. Developing countries were given a 10-year grace period, creating potential for smuggling ODS into developed countries after their 1996 ban. In response, parties adopted licensing systems to track trade and a framework requiring import/export licenses. However, illegal trade continues due to discrepancies in reported trade data, possible illicit production, weak transit controls, and lack of priority given to enforcement. As HCFC phase-outs begin, black market trade is a concern that could threaten compliance with the Protocol.