Soils constitute the largest terrestrial carbon (C) pool, representing more than the sum of carbon contained in the atmosphere and vegetation. In this context, organic soils stand out, storing 21% of the global soil organic C stock in only 3% of the Earth’s land surface. Moreover, these soils are a key component in our climate system, biodiversity, water cycle and records of environmental changes. Organic soils require specific attention as they provide a large variety of ecosystem services, but also because of their vulnerability to climate change. In this paper, we present a review of terms and concepts related to organic soils, their formation, pedoenvironments, and taxonomic classification. We also present a synthesis of studies in the highlands of southeastern Brazil using these soils as records of environmental change.