Analysis of long-term data of groundwater levels is required to provide an insight into the reaction of groundwater resources to climate variability, land use changes, human activities and consumption. In recent decades the increasing use of groundwater and natural extremes had resulted in lowering of its level in large parts of the world. In addition, it has been increasingly threatened in quantity and quality. For maintaining sustainability of groundwater resources, it is important to understand aquifer storage change in long-term. Many studies had showed under different conditions how groundwater levels change with time. This review presents long-term variations of aquifer quantity worldwide. It shows three aspects: main factors that impact groundwater level variations, applied methods to evaluate these variations and the situation of groundwater.