Agricultural landscapes of the Southern Brazilian Amazon are the result of eighty years of governmental policies to install a powerful agricultural sector. Yet, this rapid expansion raised important environmental considerations especially with regard to deforestation. The agricultural frontier is thus now facing a huge challenge: to combine socio-economic development with environmental conservation in a context of frontier expansion. Based on a conceptual model of the agricultural frontier, we review historical changes in environmental and development policies in the Brazilian state of Mato Grosso and emphasize their ambivalent trend to both encourage and control the progress of the frontier. We then extend this model with an integration stage where environmental governance and economic development evolve from competing to complementary concepts. At this stage, the efforts to slow down deforestation are accompanied with programs to promote new agricultural practices and support industrialization. Finally, we put into perspective this recent evolution with regards to the underlying reasons for changing the agricultural model, thus considering the agricultural frontier to be at a tipping point where first positive results need to be confirmed in spite of an unstable economic and political situation.