Zef has been a lawyer since 1998. He is co-founder and partner at SteensmaEven.

Zef works across all major areas of labor law. He focuses on European and collective labor law (collective labor agreements / collective dismissals / employee participation). He has extensive experience with reorganizations, corporate governance, and business transfers.

Zef is a professor of European Labor Law at Erasmus University Rotterdam. In his inaugural lecture in 2018, he addressed the potential tension between the economic opportunities European law provides to companies and employees and the social protection national labor law seeks to ensure. He obtained his PhD in 2008 on cross-border collective labor agreements in Europe.

In addition to academic teaching, Zef provides post-graduate education, including at the Grotius Academy and PALA. He regularly delivers lectures on various labor law topics, both within and outside the Netherlands.

Zef is editor-in-chief of the international journal European Employment Law Cases. He is also an editor of Tijdschrift Arbeidsrecht in Context.

Zef regularly publishes in labor law journals and books. He is co-author of the authoritative handbook Arbeidsrechtelijke Themata. Zef contributes to shaping the labor law of the future through his work on the Wetboek van Werk 2025. He served as a ‘national expert’ for the European report Cross-border Collective Actions in Europe: A Legal Challenge and participated in the evaluation of the Dutch Wet Aanpak Schijnconstructies by writing an expert report for the Ministry of Social Affairs and Employment.

Zef is a board member of the Dutch Association of Labor Law Attorneys, a member of the Rotterdam Association of Labor Law Attorneys, and of the European Employment Lawyers Association.