Water Rights in water law refers to the right of a user to use water from a water source, e.g., a river, stream, pond or source of groundwater. In areas with plentiful water and few users, the management of water rights are generally not complicated or contentious. In other areas, especially in more arid areas places like Central Oregon where there are a number of competing demands for water, management is often the source of conflict. Some states' water rights management treats surface water and ground water in the same manner, while others use different principles for each.

In the United States, there are two divergent systems for determining water rights. "Riparian" water rights (derived from English common law) are common in the eastern part of the United States and "Prior Appropriation Water Rights" (developed in Colorado and California) are common in the western part of the United States. Each state has its own variations on these basic principles, as informed by custom, culture, geography, legislation and case law. California law, for example, includes elements of both systems. In general, a water right is established by obtaining an authorization from the state in the form of a water right permit. A legal right is formally consummated, or perfected, by exercising the water right permit and using the water for a beneficial purpose.

Under the prior appropriation doctrine, water rights are "First in Time, First in Right". That is, the older, or senior, water right may operate to the exclusion of junior water rights. The concept of "Priority Date" is significant.

Water Rights are generally established pursuant to State law, but there are exceptions, most notably, the concept of federal reserved water rights. Reserved water rights are rights that are established when the federal government reserves land for a specific federal purpose. Courts have held that there is an implied water right to satisfy the primary purposes of the reservation. Examples of reservations include Indian reservations, national wildlife refuges, federal forests and military bases.

Proceedings to determine the relative priority of claims to water rights are known as adjudications. Through Congress's passage of the McCarren amendment, the federal government has consented to having its claims adjudicated in state courts.

All states offer mechanisms for changing how a water right is exercised, e.g., amending the point of diversion or withdrawal, the place of use and the purpose of use. In reviewing such requests, the state must guard against the impairment of other water rights, the enlargement of the water right and injury to the public interest.