Return flow to groundwater is the quantity of water applied at or near the land surface which infiltrates back (returns) to the groundwater system. Common uses that lead to return flow are irrigation of agricultural fields, golf courses or lawns, domestic wastewater disposal through septic systems, and artificial recharge. Quantifying the amount of this applied water that percolates to the water table is necessary in many water-short western states to prove beneficial use, evaluate return flow credits, demonstrate the portion of a water right which can be transferred to another party, and for water banking computations in aquifer storage and recovery projects. Return flow analysis is also relevant to municipal water reuse projects, where the impacts to groundwater quality from landscape and golf course irrigation or artificial recharge with treated wastewater are a potential concern. The purpose of this paper is to describe return flow processes and methods available to quantify return flow to groundwater. The first part of the paper sets the statutory and regulatory contexts for return flow analyses. The second part of the paper deals with a review of the available methods to quantify return flow to groundwater. Although a very common method is the water balance approach applied at the point of use, with the corresponding assumption that the residual water component becomes deep percolation (return flow), we emphasize the less commonly used approaches that rely on data and computations in the vadose zone and in the aquifer.