Groundwater flows in the pores and fractures of soils and rocks, driven by changing mechanical and environmental boundary conditions, for example air temperature, humidity, and precipitation. Because the groundwater environment continuously changes, the steady--state condition is only a theoretical concept: the movement of water in soils and rocks is transient, and any change is delayed with respect to the trigger. This chapter focuses on the role of time and the effects of anthropogenic, natural, and climatic processes on groundwater flow and slope behavior. The unsaturated condition is considered a normal soil state and the saturated condition a special case.