In 2000, the Dutch authorities lifted the ban on brothels in the Netherlands. The essence of their approach was to regulate prostitution. People of legal age could now voluntarily sell and purchase sexual services. Brothels which complied with certain licensing conditions were legalized. This paper critically assesses the logic of a position that argues that human trafficking is reduced when actors in the legalized prostitution sector are made responsible for what happens on their premises (using licensing conditions). This idea is confronted with empirical evidence about the Netherlands in general and the city of Amsterdam in particular. Furthermore, the paper addresses two questions. What are consequences of the regularization of prostitution for the criminal investigation and prosecution of sex trafficking? How do criminal justice agencies collaborate with regulatory authorities in the regulated and non-regulated sectors of the prostitution market? The main conclusion is that the screening of brothel owners and the monitoring of the compliance of licensing conditions do not create levels of transparency that enable sex trafficking to be exposed. The prostitution business retains many characteristics of an illegitimate market and the legalization and regulation of the prostitution sector has not driven out organized crime. On the contrary, fighting sex trafficking using the criminal justice system may even be harder in the legalized prostitution sector.