Ostrom was the first woman to win the prize since it was founded in 1968, and the fifth woman to win a Nobel award this year _ a Nobel record.
Ostrom, who has devoted her career to studying the interaction of people and natural resources, told the Royal Swedish Academy of Sciences by telephone that she was surprised by the Nobel.
"There are many, many people who have struggled mightily and to be chosen for this prize is a great honor," she said. "I'm still a little bit in shock."
The academy cited Ostrom "for her analysis of economic governance," saying her work had demonstrated how common property can be successfully managed by groups using it.
Williamson, the academy said, developed a theory where business firms can serve as structures to resolve conflicts.
"Over the last three decades, these seminal contributions have advanced economic governance research from the fringe to the forefront of scientific attention," the academy said.
It said the American winners' research shows that economic analysis can shed light on most forms of social organization.
"Elinor Ostrom has challenged the conventional wisdom that common property is poorly managed and should be either regulated by central authorities or privatized," the academy said. "Based on numerous studies of user-managed fish stocks, pastures, woods, lakes, and groundwater basins, Ostrom concludes that the outcomes are, more often than not, better than predicted by standard theories."
One notable publication Ostrom wrote in 1990 examined both successful and unsuccessful ways of governing natural resources _ forests, fisheries, oil fields, grazing lands and irrigation system _ that are used by individuals.
Williamson was cited for arguing that markets and hierarchical organizations, such as firms, represent alternative governance structures that differ in their approaches to resolving conflicts of interest.
Issues of governance, or the rules by which authority is exercised in companies and economies, have been at the heart of the ongoing world economic crisis. The failure by boards of directors, for instance, to police excessive compensation, or prevent bonuses that reward excessive risk taking, can be considered a corporate governance issue.
The economics prize was the last Nobel award to be announced this year. It's not one of the original Nobel Prizes, but was created by the Swedish central bank in Alfred Nobel's memory.
Nobel Prize winners receive 10 million Swedish kronor ($1.4 million), a gold medal and diploma from the Swedish king on Dec. 10, the anniversary of Nobel's death in 1896.
In the biggest surprise of this year's awards, President Barack Obama won the Nobel Peace Prize on Friday.
In other awards, American scientists Elizabeth H. Blackburn, Carol W. Greider and Jack W. Szostak shared the Nobel Prize in medicine for discovering a key mechanism in the genetic operations of cells, an insight that has inspired new lines of research into cancer.
The physics prize was split between Charles K. Kao, who helped develop fiberoptic cable, and Americans Willard S. Boyle and George E. Smith who invented the "eye" in digital cameras.
Americans Venkatraman Ramakrishnan and Thomas Steitz and Ada Yonath of Israel shared the chemistry prize for their atom-by-atom description of ribosomes.
Romanian-born German writer Herta Mueller won the literature prize.
AP Writers Malin Rising in Stockholm and Jeannine Aversa in Washington contributed to this report.
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