Individuals, organisations, populations: at what level(s) should we collect, analyse and interpret data? How can we envision the articulation between these different levels if we choose more than one? These questions arise in all research, but sometimes in an almost unconscious or mechanical way. This introduction outlines a number of milestones that will enable to place these questions in their historical context, to specify what we are talking about when we refer to these micro-meso-macro levels, and to put forward some of the main issues at stake in the articulation between these levels.