The article presents the first systematic comparative study on the growing involvement of international organizations in national constitution making around the world. Over the past three decades, the emerging field of international constitutional advising has undergone an intense process of institutionalization and professionalization, mirroring the increasing role constitution making is playing in both national and international politics. Despite the vast scope of the phenomenon, the involvement of foreign constitutional advisors in domestic constitution-drafting or constitutional reforms has received little scholarly attention. This article takes the first steps towards addressing this lacuna empirically, by introducing a new dataset on 46 international organizations involved in 730 constitutional advising projects in 145 countries between 1989 and 2017. We classified the organizations based on their type, their headquarters’ location, the countries they target, the kind of advising activities they perform and the level of directness of the advising intervention. While generally, we find a significant correlation between more direct constitutional advising activities and larger relative changes in the quality of democracy and larger numbers of constitutional systems in a country, the article suggests avenues for more nuanced research to better understand constitutional advising’s impact.