A lawyer and historian, he has been investigating human rights violations in different countries for nearly three decades. After serving as U.S. Attorney for the Southern District of New York, he joined Humans Rights Watch (HRW) in 1987 and became its executive director in 1993. 

Over almost three decades, he has led a process of expansion and transformation of HRW, now present in more than 90 countries, making it an international reference entity recognised in 1997 with the Nobel Peace Prize. During his mandate, the organisation has scrupulously documented numerous human rights violations in war contexts in countries such as Rwanda, Syria, Yemen, Ukraine and Afghanistan, and in situations of peace and democracy, such as the torture of terrorist suspects in the United States.

Through his work at HRW, he has contributed to the transformation of the international peace landscape, professionalising and making visible the work of documenting human rights abuses. Throughout his vast career, he has published more than a hundred articles in specialised journals and has contributed on occasion to publications such as The Guardian, The New York Review of Books and The Washington Post.