Introduction <p>Scaling up public health interventions is a major avenue for evidence-based public health policy. Many attempts have been made, but there have also been major difficulties and many failures. Scaling up is not easy; it is neither obvious nor spontaneous.</p> Objectives <p>This study sets out to clarify the concept itself, distinguish the strategies on which it is based, describe the process, and identify the factors that affect it.</p> Methods <p>Semi-structured interviews were conducted with 27 scale-up experimenters from a variety of backgrounds. The interviews focused on 19 public health interventions.</p> Results <p>Scaling up is an active, progressive, and multidimensional process. It is based on a variety of strategies relating to territorial expansion, sustainability, and adaptation to the realities of the intervention. Four types of favourable decision could be identified: funding, support, commitment, and adoption; they emanate from multiple decision-makers. Eight essential activities emerged. In addition, scaling is based on organisations that adopt different conformations and different methods of identifying multipliers. Finally, six catalysts and six inhibitors were identified.</p> Conclusion <p>This study has made it possible to stabilise the concept of scaling up and deepen our understanding of the process. New opportunities are emerging to support the players and decision-makers involved in scaling up public health interventions. Similarly, new research perspectives are emerging, in particular through the mobilisation of the political sociology of public action, and management and marketing sciences.</p>