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The concepts of preferential flow are reviewed. Five types of preferential flow are recognized and defined. These include macropore flow, gravity-driven unstable flow, heterogeneity-driven flow, oscillatory flow, and depression-focused recharge. Spatial scales at which each of these preferential flow processes occurs are suggested. An overview is presented of the relationship between preferential flow and water quality, both for surface water sources and ground water sources. Much of the direct knowledge about the impact of preferential flow on water quality has been derived within the last two decades. The evidence for the impact of preferential flow on surface water quality is much more plentiful, probably because it is easier to measure chemical breakthroughs to surface water sources. Some of the newer methods for experimentally quantifying preferential flow are outlined. These methods include the tension infiltrometer, time domain reflectometry, chemical tracing, and geophysical methods such as ground penetrating radar and electrical resistance tomography. Theoretical analyses of preferential flows are based on conservation of mass and momentum. It is explained that the use of Darcy's law (and therefore Richards' equation) may not be valid for some preferential flows.