Quantifying natural recharge to aquifers is becoming increasingly important in the Southwest. Many ground-water management decisions are based on the amounts of recharge to aquifers, and knowledge of location, timing, and amount of recharge is critical to scientists studying ground-water movement. Recharge to aquifers has been defined as water crossing the water table from the unsaturated zone. For those who manage water resources, however, a broader definition including all water that enters aquifers is of more practical use. Additional components of recharge include water entering aquifers from surface water in direct hydraulic connection with the underlying ground water and water moving into aquifers from adjacent rock units. Because water enters aquifers in various ways and over large areas, recharge cannot be measured directly. Furthermore, recharge varies with time in response to climate variations. Hydrogeologists commonly estimate single long-term average recharge rates for aquifers, but additional insight could be gained by understanding effects that climate variations of different time scales have on recharge. Current studies are developing methods to better estimate natural ground-water recharge, but many of these methods are applicable only at site scales. Determining recharge for entire aquifer systems likely will remain difficult, requiring investigations that use multiple approaches and various scales.