Waste stabilization ponds (WSPs or stabilization ponds or waste stabilization lagoons) are ponds designed and built for wastewater treatment to reduce the organic content and remove pathogens from wastewater. They are man-made depressions confined by earthen structures. Wastewater or "influent" enters on one side of the waste stabilization pond and exits on the other side as "effluent", after spending several days in the pond, during which treatment processes take place.

Waste stabilization ponds are used worldwide for wastewater treatment and are especially suitable for developing countries that have warm climates. They are frequently used to treat sewage and industrial effluents, but may also be used for treatment of municipal run-off or stormwater. The system may consist of a single pond or several ponds in a series, each pond playing a different role in the removal of pollutants. After treatment, the effluent may be returned to surface water or reused as irrigation water (or reclaimed water) if the effluent meets the required effluent standards (e.g. sufficiently low levels of pathogens).

Waste stabilization ponds involve natural treatment processes which take time because removal rates are slow. Therefore, larger areas are required than for other treatment processes with external energy inputs. Waste stabilization ponds described here use no aerators. High-performance lagoon technology that does use aerators has much more in common with the activated sludge process. Such aerated lagoons use less area than is needed for traditional stabilization ponds and are also common in small towns.

Sewage and many types of industrial wastewaters contain organic matter. If wastewater is discharged untreated into surface water bodies (for instance, rivers and lakes), their organic matter serves as food for microorganisms living in the surface waters. These organisms use the organic matter for energy generation for their growth and reproduction. This is done via their respiration, in which they convert the organic matter into carbon dioxide and water. These stable components do not cause water pollution problems. Therefore this is frequently called "stabilization" of the organic matter.^{[citation needed]}

However, these organisms use oxygen in their respiration, thus reducing the oxygen concentration in the surface waters. This is one of the main water pollution problems, which may affect the surface water biota, including fish.

Waste stabilization ponds reproduce these biological phenomena before they take place in the receiving surface water and cause the pollution problems due to oxygen consumption. The ponds receive wastewater, and, by natural processes similar to those that take place in the surface waters, carry out stabilization of the organic matter inside them, as part of the treatment. This is why they received the name of waste stabilization ponds.

The reactions take place by the joint participation of several microorganisms living within the pond. The organic matter is measured as biochemical oxygen demand (BOD). BOD values in the pond effluent are lower than in the influent, reflecting the removal of organic matter. This pond biome uses organic matter from the wastewater as food.^{[citation needed]}

Nutrients are converted to cell material and energy for life processes including reproduction and growth of living cells. Some of these living cells will be consumed by organisms at higher trophic levels within the pond. In ponds, the most important group of microorganisms are bacteria, which utilize most of the organic matter from the wastewater, but also consume oxygen. ^{[citation needed]}