In 2011, the United Nations Guiding Principles on Business and Human Rights (UNGPs) established that business enterprises should exercise human rights due diligence (HRDD) as part of their responsibility to respect human rights. HRDD requires companies to undertake processes to identify, prevent, mitigate and account for how they address potential and actual impacts on human rights. Following the adoption of the UNGPs, corporate HRDD has become a norm of expected conduct. In Europe, an increasing number of countries have adopted or taken steps to adopt legislation imposing mandatory HRDD on businesses. In this context, Didier Reynders, the Commissioner for Justice, recently announced that the European Commission would introduce legislation to impose mandatory HRDD on companies by 2021. This announcement was a turning point in the field of business and human rights. If adopted, this European Union (EU) instrument would be the first regional legislation to translate HRDD from a concept into hard law. Given the economic and political leverage of the EU, such an instrument could have a significant impact on respect of human rights by companies around the world. It would also be a fundamental change in the way the EU sees its role in ensuring corporate accountability for human rights. In addition, an EU mandatory HRDD instrument could encourage countries in other parts of the world to adopt similar standards. This contribution discusses how a mandatory EU HRDD instrument could effectively improve corporate accountability for human rights. First, it considers the legal value-added of a mandatory EU HRDD instrument. It maps the current EU legislative and policy framework on business and human rights and identifies the gaps that justify the adoption of HRDD norms at EU level. Second, this contribution examines potential options to ensure that the EU mandatory HRDD instrument fills these gaps. Provisions on scope of application, enforcement mechanisms, business due diligence obligations and access to justice and remedies will have a significant impact on this instrument's effectiveness in protecting human rights. Nevertheless, they will be the most difficult to negotiate. Ultimately, this contribution argues that a number of substantive and political barriers, such as business lobbying against strong mandatory HRDD standards, risks undermining the potential value-added of the EU mandatory HRDD instrument.