In many European countries, the study of the long-term future has often been a concern for public authorities. Several tools allow to explore the future(s) and this PhD thesis focuses on one of them: foresight. It is understood as a long-term systemic and interdisciplinary policy-making tool aiming at identifying possible long-term futures and including trends disruptions and surprises. This PhD dissertation addresses the institutionalization of foresight activities within public policy. Analyzing institutionalization as an ongoing process, it compares such processes in two policy sectors (health policy and energy policy) within three politico-administrative systems (the United Kingdom, the Netherlands and the Walloon Region). It aims at identifying factors of institutionalization of such practices within public policy. In this respect, the two first chapters define the key concepts of the research, i.e. foresight, institutionalization of foresight and politico-administrative systems, while the third one presents the epistemological and methodological aspects of the dissertation. In that regard, the PhD is based on two broad literature reviews respectively considering foresight and futures studies in the one hand and institutional theory on the other hand. In addition, it is based on a large set of collected data, including more than 1200 public documents, 84 semi-structured interviews and hours of participant observation in a Walloon public agency notably in charge of foresight. These data are stored, analyzed and retrieved thanks to a computer-assisted qualitative data analysis software (NVivo). The chapters 4, 5 and 6 respectively cover processes of institutionalization of foresight in the United Kingdom, in the Netherlands and in the Walloon Region, starting from the 1970s up to 2010 (for the two former ones) and up to 2015 (for the latter). Each of them highlight several approaches to foresight within a single politico-administrative system as well as a variety of processes of institutionalization. In that ...