Rainwater harvesting means collecting the rain that falls on-site (usually on roofs). This method is mainly used for irrigation, flushing, or other uses, although if properly treated, it can also be used for drinking water. Rainwater harvesting is a valuable way to reduce or even eliminate the use of municipal drinking water, without requiring individuals to cut back on water consumption. This solution is, of course, more effective in rainy climates than in dry ones.

Rainwater harvesting systems are measured by the area for collecting water (in square meters or square feet) and the volume of water they store (in liters or gallons).

Simple rainwater harvesting systems have three main elements: the roof or other catchment surfaces, storage tank(s), gutters, and other piping that directs water from the catchment surface to the tank. Advanced systems may use a pump to draw water from the tank to the point of use and purify the water with additional devices like filters and ultraviolet disinfectants.

If rainwater is used for drinking or garden irrigation, ensure the tank does not leach toxins or promote pathogen growth. For example, galvanized steel tanks lined with polyethylene or other food-grade liners are suitable.

If rainwater collected from the roof is used for drinking or garden irrigation, ensure the roofing materials do not leach toxins. For instance, asphalt shingles leach toxins into the water, while metal roofs or slate shingles do not.