This chapter studies the joint efforts of states to tackle crime through bilateral or multilateral action. The criminal activities that fuel global concern are transnational in nature; they involve more than one country and thus require an internationally coordinated response. These transnational criminal activities involve illicit flows; that is, the movement between countries of people, goods, or money. What makes these flows illicit is that they are prohibited by the laws of the country that is the source of the flows and/or the laws of the country receiving them. Smuggling drugs or firearms across borders; laundering illegally obtained funds through international financial transactions; the sale of women to engage in sex work—these are some examples of the transnational flows that constitute the illicit global economy. The chapter examines each of these flows, the challenge of measuring them, and their relationship with globalization, before turning to the efforts against them.