The term “Paleosol” refers to a fossil soil, or remnants thereof, either preserved at modern land surface or buried by layers of younger sediment. The study of Paleosols is referred to as paleopedology. Paleosols preserve evidence of past physical and chemical weathering (such as reddening due to oxidation of iron), macro-biological activity (such as burrowing and additions of organic matter), microbial activity (reduction of iron or gleying), and/or additions of younger sediment and the depositional processes associated with them. Modern soil characteristics and processes, and their relationship to the contemporary environment, are the “keys to the past” and provide the basis for interpreting past climate, vegetation, and other environmental conditions from Paleosols. Paleosols are also a primary component of allostratigraphy, a branch of stratigraphy that uses unconformities and their cross-cutting relationships for correlating sedimentary successions. Paleosols that serve as allostratigraphic marker beds are more formalized and thus are referred to as Geosols. Challenges for future research on Paleosols include seeking improved field and laboratory methods to differentiate the original from the diagenetically altered characteristics, due to postburial change, as well as autochthonous from allochthonous materials. For example, many changes in Paleosol profile chemistry and mineralogy classically have been attributed to in situ mineral weathering. These concepts are challenged by the relatively recent discovery of dust accumulations in surface soils. Answering questions of this sort often requires the use of mass balance techniques (including trace elements and isotopes) and micromorphology. Continued advances in dating methods, particularly luminescence techniques, will provide better temporal context for many Paleosols and associated sediments. Collectively, improved analytical chemistry and chronology will improve our assessment of the characteristics of soil-forming intervals, and answer the longstanding question as to whether rates of soil formation were faster during optimum conditions in the past.