This chapter considers the baseline as a social construct calls for the exploration of at least three lines of thought. First, the baseline is not stabilized but evolves over time. Second, different elements, not only ecological, but also social, cultural, economic and even political, enter into the construction of the baseline, hence the fact that the baseline may vary according to social groups or individuals, and to contexts. Finally, since the choice of the baseline has strong management implications and there is not necessarily a consensus, it is necessary to debate it. The shifting baseline syndrome theory has met with widespread acceptance in the scientific community. If we accept that the baseline is a social construct, it remains to be understood how this construction takes place. The baseline has a descriptive value: it gives an image of the past state of the ecosystems, and makes it possible to give meaning to the dynamics in progress.