Tom Zwart focuses on the interplay between law and culture, which includes religion. His research includes the following questions: Which value does the local culture accord to formal state law? Which other legal systems are recognised by local culture? Does local culture act as a building block or a stumbling block for compliance with formal state law? Which - invisible - cultural assumptions underlie formal state law? Which role is assigned to culture in the area of comparative law?

Zwart focuses in particular on human rights, especially concepts and ideas developed in the Global South, and the mutual acculturation of minorities, especially Muslims, and the societies of which they are part. 

In the field of human rights Zwart developed the so-called receptor approach to human rights, which regards culture and religion as building blocks rather than stumbling blocks for human rights protection and promotion. His reserach in this area was awarded with a 1 million Euro grant by the Dutch Foreign Office. 

Zwart is the founder and the secretary-general of the Cross-cultural Human Rights Network, which is aimed at familiarising Northern audiences with Southern ideas, concepts and theories on human rights. The Network counts scholars and universities from across the world as its members, especially from the Global South.

In the area of the acculturation of minorities and the societies of which they are part Zwart pays particular attention to the mutual acceptance of Muslim minorities and majorities in societies in Western-Europe and China. This mutual acceptance is being determined in part by Islamic law, which is included in his research. 

Zwart is a registered expert at the ICC on forensic ethnology, i.e. the cultural dimensions of war crimes and crimes against humanity.

The law students’ societies of both Leiden and Utrecht have appointed Zwart an honorary member for his excellence in teaching and his commitment to students.

Zwart has been elected a visiting professor at a number of universities, including the University of Cambridge, L’Institut d’Études Politiques (Sciences-Po) in Paris, and Tsinghua University in Beijing.