On the basis of research conducted in three contrasting South American ecoregions (southern Patagonia, Argentina; the central Pampas and Campos, Uruguay; and western Amazonia, Brazil), this chapter aims to better understand the complex sets of reasons that have recently led local societies to adopt more sustainable pasture management in South America. After a brief overview of each of the three biomes, representative social–ecological systems of pastoralism are identified with the objective to describe their respective pasture management history, especially the colonization process and the settlement of the pioneers, and the successive farming systems, mainly the practices related to herd and pasture management, are compared. Finally, the main local and national policies regarding the livestock sector and landownership are analyzed. The evolution of the social–ecological systems of pastoralism in these ecoregions was assessed with a three-dimensional model of vulnerability based on the agroecosystem resilience, livelihood improvement, and institutional capability. The evolution of the mental models about livestock has also been analyzed to better understand the current perceptions of the local people and their scenarios for the future of livestock in their social–ecological systems. The results focused on the dynamics of rangeland management, the vulnerability of the pastoral social–ecological systems, and the evolution of the mental models of local people about livestock, so as to discuss the different rhythms of transformation and the existence of critical thresholds. Besides diverse land and social issues, it is imperative to intensify livestock production and increase the offer of new technologies, as well as to identify some relevant human dimension factors, such as the demand of local societies for collective livelihood improvement, the recent national and international environmental policies related to global warming and biodiversity, and the stronger awarene