Over 40 million people around the world are victims of modern forms of slavery: forced labour and human trafficking. People are tricked into working under onerous conditions, and unable to leave or return home due to physical, psychological or financial coercion, and many of these trafficking victims produce goods for United States (US) and other multinational corporations that profit by relying on the lower wages earned by workers in their global supply chains. Well-developed legal standards prohibit these practices, and governments, intergovernmental organizations, business associations and non-governmental organizations have developed mechanisms to prevent, detect and provide redress to victims. Some businesses lead or comply with the standards and enforcement mechanisms, but too many do not. US law offers a powerful but under-utilized tool to address trafficking: the 2008 Trafficking Victims Protection Reauthorization Act (TVPRA), which imposes civil liability on those who ‘knew or should have known’ about forced labour or human trafficking in their corporate ventures. Unfortunately, courts have ignored or misinterpreted this standard, at times confusing civil and criminal provisions of the statute. Correct and vigorous legal enforcement is key to addressing the accountability gap between the well-developed standards and the continuing use of forced labour and human trafficking. This article is the first to demonstrate that, with regard to the TVPRA standard, corporations have long been on notice of both the obligation to effectively monitor labour conditions and the mechanisms that would accomplish that task. US courts must enforce the ‘knew or should have known’ standard to protect workers – the most vulnerable people in the supply chain – and to prevent an unfair competitive advantage over companies that have established compliance programmes that actually prevent and punish human trafficking and forced labour.