  Land use expansion is linked to major sustainability concerns including
climate change, food security and biodiversity loss. This expansion is largely
concentrated in so-called frontiers, defined here as places experiencing marked
transformations due to rapid resource exploitation. Understanding the
mechanisms shaping these frontiers is crucial for sustainability. Previous work
focused mainly on explaining how active frontiers advance, in particular into
tropical forests. Comparatively, our understanding of how frontiers emerge in
territories considered marginal in terms of agricultural productivity and
global market integration remains weak. We synthesize conceptual tools
explaining resource and land-use frontiers, including theories of land rent and
agglomeration economies, of frontiers as successive waves, spaces of
territorialization, friction, and opportunities, anticipation and expectation.
We then propose a new theory of frontier emergence, which identifies exogenous
pushes, legacies of past waves, and actors anticipations as key mechanisms by
which frontiers emerge. Processes of abnormal rent creation and capture and the
built-up of agglomeration economies then constitute key mechanisms sustaining
active frontiers. Finally, we discuss five implications for the governance of
frontiers for sustainability. Our theory focuses on agriculture and
deforestation frontiers in the tropics, but can be inspirational for other
frontier processes including for extractive resources, such as minerals.
