The introduction of this issue devoted to the history of coastal environments proposes to go beyond the ‘great divide’ between land and sea, which still largely shapes the field of environmental history, and to rethink their interactions within the framework of a ‘terraqueous’ environmental history. The coasts are a privileged terrain for tackling this dichotomous vision. Because they are the hybrid product of natural and social dynamics, their study allows us to historicise and contextualise the construction, both materially and symbolically, of the existing boundaries between land and sea. It furthermore allows us to show the always liminal, shifting and permeable character of the coast, which functions more as an interface than as a border. Finally, because of their position on the edge, coasts have a ‘mirroring’ power which invites us to pay more attention to the symmetry between land and sea.