The Pacific Proving Grounds was the name given by the United States government to a number of sites in the Marshall Islands and a few other sites in the Pacific Ocean at which it conducted nuclear testing between 1946 and 1962. The U.S. tested a nuclear weapon (codenamed Able) on Bikini Atoll on June 30, 1946. This was followed by Baker on July 24, 1946 (dates are Universal Time, local dates were July 1 and 25, respectively).

On July 18, 1947, the United States secured an agreement with the United Nations to govern the islands of Micronesia as the Trust Territory of the Pacific Islands, a strategic trusteeship territory. This is the only such trusteeship ever granted by the United Nations to the United States. The Trust Territory comprised about 2,000 islands spread over 3,000,000 square miles (7,800,000 km²) of the North Pacific Ocean. Five days later, the United States Atomic Energy Commission established the Pacific Proving Grounds.

The United States conducted 105 atmospheric and underwater (i.e., not underground) nuclear tests in the Pacific, many with extremely high yields. While the Marshall Islands testing composed 14% of all U.S. tests, it composed nearly 80% of the total yields of those detonated by the U.S., with an estimated total yield of around 210 megatons, with the largest being the 15 Mt Castle Bravo shot of 1954 which spread considerable nuclear fallout on many of the islands, including several that were inhabited and some that had not been evacuated.

Many of the islands which were part of the Pacific Proving Grounds are still contaminated from the nuclear fallout, and many of those who were living on the islands at the time of testing have suffered from an increased incidence of various health problems. Through the Radiation Exposure Compensation Act of 1990, at least $759 million has been paid to Marshall Islanders as compensation for their exposure to U.S. nuclear testing. Following the Castle Bravo accident, the U.S. paid $15.3 million to Japan.

Scientists have calculated that the residents of the Marshall Islands during their lifetimes will be diagnosed with an added 1.6% (with 90% uncertainty range 0.4% to 3.4%) cancers attributable to fallout-related radiation exposures. The cancers are the consequence of exposure to ionizing radiation from weapons test fallout deposited during the testing period (1948–1958) and from residual radioactive sources during the subsequent 12 years (1959–1970).