Playas may form in both hydrologically open and closed systems. Several previous classification schemes of playas rely heavily on geomorphic features rather than the groundwater hydrology of the system and the degree to which the basin is hydrologically open or closed. This paper presents a review of the literature on playa classifications and their hydrologic characteristics, which can be used to better define the hydrology of playas. A modification of the original hydrologically based classification schemes of Meinzer (1922) and Snyder (1962) illustrates the importance of groundwater in playa systems. In this paper a playa is defined as an intracontinental basin where the water balance of the lake (all sources of precipitation, surface-water flow, and groundwater flow minus evaporation and evapotranspiration) is negative for more than half the year, and the annual water balance is also negative. The playa surface must act as a local or regional discharge zone. Evidence of evaporite minerals will generally be present in parts of the basin. This includes carbonate minerals that can be demonstrated to have been formed through evaporative processes. A recharge playa is defined as above, except the playa surface acts as a means for recharging water to the aquifer. In this case, evaporite minerals are absent. Although evaporite formation is an important part of a playa sequence, significant accumulations of subaqueously deposited evaporites are only possible when saline water bodies are partially maintained by a constant inflow of groundwater. In hydrologically closed basins, the brine chemistry is influenced by the lithology of the sediments and bedrock within the playa catchment. In hydrologically open (through-flow) discharge complexes, the brine composition may be determined by the rate of groundwater flow through the basin (leakage ratio) relative to the weathering rate of the surrounding sediments and bed rock. If the groundwater inflow greatly exceeds the rate of chemical weathering, then the brine composition may be dominated by the chemical composition of regional or local precipitation. Extensive accumulations of subaqueous evaporites form in playas when there is sufficient groundwater to maintain a shallow brine pond for an extended period of time. This has important implications for paleoclimate reconstructions of closed basin playa sequences. Extremely arid periods, when there is insufficient water to maintain a brine, will result in displacive intrasediment growth of evaporites; and extremely wet periods may be too fresh for a brine to develop. It is only in the intermediate periods when evaporation is high and water input is balanced by evaporation that extensive subaqueous evaporites will accumulate in closed basin playas.